Glossary
Reflection of Light
Definition: The phenomenon where light bounces back when it strikes a polished or shiny surface.
Laws of Reflection:
First Law: The incident ray, reflected ray, and the normal (at the point of incidence) all lie in the same plane.
Second Law: The angle of incidence (∠i) is equal to the angle of reflection (∠r).
- Refraction of Light
Definition: The bending of light when it passes obliquely from one transparent medium to another due to a change in its speed.
Cause: Light travels at different speeds in different media (e.g., faster in air, slower in water or glass).
- Incident Ray
Definition: The incoming light ray that strikes a surface.
- Refracted Ray
Definition: The light ray that bends as it enters a new medium.
- Emergent Ray
Definition: The light ray that exits a medium after refraction.
- Normal
Definition: An imaginary line perpendicular to the surface at the point where light strikes.
- Angle of Incidence (∠i)
Definition: The angle between the incident ray and the normal.
- Angle of Refraction (∠r)
Definition: The angle between the refracted ray and the normal.
- Optical Density
Definition: A measure of how much a medium slows down light. A higher optical density means light travels slower.
- Refractive Index (μ)
Definition: The ratio of the speed of light in vacuum (or air) to its speed in a given medium.
Formula:
u =
- Optically Denser Medium
Definition: A medium where light travels slower (higher refractive index). Example: Glass, water.
- Optically Rarer Medium
Definition: A medium where light travels faster (lower refractive index). Example: Air, vacuum.
- Laws of Refraction (Snell’s Law)
First Law: The incident ray, refracted ray, and normal lie in the same plane.
Second Law: The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant (refractive index).
Formula:
- Dispersion of Light
Definition: The splitting of white light into its constituent colors (VIBGYOR) due to different refractive indices for different wavelengths.
Example: Rainbow formation.
- Prism
Definition: A transparent optical element with flat, polished surfaces that refract light, causing dispersion.
- Lens
Definition: A transparent optical device (usually glass or plastic) that refracts light to converge or diverge rays.
- Convex Lens (Converging Lens)
Definition: A lens that is thicker at the center and converges light rays to a focal point.
Uses: Magnifying glasses, cameras, eyeglasses for hypermetropia.
- Concave Lens (Diverging Lens)
Definition: A lens that is thinner at the center and diverges light rays.
Uses: Correcting myopia (nearsightedness).
- Principal Axis
Definition: An imaginary straight line passing through the centers of curvature of a lens.
- Optical Centre
Definition: The central point of a lens where light passes undeviated.
- Focal Point (Principal Focus)
Definition: The point where parallel rays of light either converge (convex lens) or appear to diverge (concave lens).
- Focal Length (f)
Definition: The distance between the optical center and the focal point.
- Real Image
Definition: An image formed when light rays actually meet (can be projected on a screen).
- Virtual Image
Definition: An image formed when light rays appear to meet (cannot be projected on a screen).
- Magnification (m)
Definition: The ratio of the height of the image to the height of the object.
Formula:
- Total Internal Reflection
Definition: The complete reflection of light inside a denser medium when the angle of incidence exceeds the critical angle.
Applications: Optical fibers, mirages.
- Critical Angle
Definition: The minimum angle of incidence beyond which total internal reflection occurs.
- Mirage
Definition: An optical illusion caused by the refraction and total internal reflection of light in hot air layers, making distant objects appear inverted or shimmering.
- Rainbow
Definition: A natural spectrum formed by the dispersion, refraction, and reflection of sunlight in water droplets.
- Twinkling of Stars
Definition: The apparent flickering of stars due to atmospheric refraction of starlight.
Something To Know
A. Fill in the blanks.
- A ray of light, passing from one medium to another, does not bend its path if its angle of incidence equals ____________ degree.
Answer – 0 degree
- The more is the optical density of a given medium, the ____________ is the speed of light through it.
Answer – less (or slower)
- The phenomenon of the splitting of white light into seven colours is known as the phenomenon of ____________ .
Answer – dispersion
- The point, on the principal axis of a concave lens, from where a beam of incident parallel ray appears to diverge, is called the ____________ of the concave lens.
Answer – principal focus (or focal point)
- An object should be placed at the ____________ point so that a convex lens forms its real and inverted image of the same size.
Answer – 2F (twice the focal length)
B. State True or False for the following statements.
- The basic cause of refraction is the change in the speed of light as it goes from one medium to another.
Answer – True ✅
(Refraction occurs because light changes speed when moving between media of different optical densities.)
- In an optically denser medium, the speed of light is more than the speed of light in vacuum.
Answer – False ❌
(Light travels fastest in vacuum (~3×10⁸ m/s). In denser media (e.g., glass, water), its speed decreases.)
- An (obliquely) incident ray always bends away from the normal when it passes from one transparent medium into another.
Answer – False ❌
(It bends toward the normal when entering a denser medium and away from the normal when entering a rarer medium.)
- A coin, kept at the bottom of an empty dry cup, appears to ‘rise up’ when some water is poured into the cup.
Answer – True ✅
(Due to refraction, light rays from the coin bend away from the normal as they exit water, making the coin appear higher.)
- When white light is ‘dispersed’ by a glass prism, the yellow colour in it, bends more than the blue colour.
Answer – False ❌
(Blue light bends more than yellow because it has a higher refractive index and slower speed in glass. Violet bends the most, red the least.)
C. Tick (✓) the correct option.
- Which of the following conditions is not necessary for a change in the direction of propagation when a light ray goes from one medium to another?
The light ray must be incident at an angle greater than 0° with respect to the normal at its point of incidence.
The two media must both be transparent.
The two media must have different refractive indices.
The incident light rays must always have only the blue colour.
Answer – The incident light rays must always have only the blue colour.
(Refraction occurs for all colours; the colour of light does not affect whether refraction happens.)
- Which of the following diagrams correctly shows the ‘bending of a light ray’ as it goes from an optically denser medium into an optically rarer medium?
Answer – ✅ The light ray bends away from the normal (angle of refraction > angle of incidence).
- The angle, between the incident ray and the emergent ray, for a rectangular glass slab is—
0°
90°
an obtuse angle
a non-zero acute angle
Answer – ✅ 0°
(The emergent ray is parallel to the incident ray but laterally displaced.)
- Which of the following figures correctly represents the passage of white light through a glass prism?
Answer – ✅ Second Figure (if it shows violet at the bottom and red at the top).
Reason:
Prisms disperse light due to wavelength-dependent refraction (shorter wavelengths like violet bend more).
The first figure (if it reverses the colours) would be incorrect.
- An incident ray, passing through the optical centre of a concave lens, after refraction through it, will—
move parallel to its principal axis.
pass through its principal focus ‘F’.
go undeviated.
pass through its 2F point’.
Answer – Go undeviated.
(Rays passing through the optical centre of any lens (convex or concave) do not bend.)
- When an object approaches a convex lens, from infinity towards its focus, the image, formed by it—
keeps on shifting away from the lens.
keeps on shifting towards the lens.
shifts first towards, and then away from the lens.
shifts first away, and then towards the lens.
Answer – Keeps on shifting away from the lens.
(As the object moves from infinity to focus, the real image moves from F to infinity on the other side.)
- Tanya’s grandmother needs a lens to read small letters in her book. For this she should use—
a convex lens (of focal length F), kept at a distance between F and 2F from the book.
a concave lens, kept at a distance less than its focal length from the book.
a concave lens, kept at a distance between F and 2F, from the book.
a convex lens, kept at a distance less than its focal length from the book.
Answer – A convex lens, kept at a distance less than its focal length from the book.
(A convex lens acts as a magnifying glass when the object is placed within its focal length, producing a virtual, erect, and enlarged image.)
D. Answer the following questions in brief:
- Light propagates faster through a Medium A than through another Medium B.
(a) Which of the two media has a higher optical density?
Answer – Medium B (since slower speed = higher optical density).
(b) State the condition under which no ‘change in the direction of propagation’ of a light ray would take place when it passes from Medium A to Medium B.
Answer – If the light ray is incident normally (0° angle of incidence) to the boundary.
- Why does a pencil appear bent when it is partially dipped in water?
Answer – Light rays from the submerged part bend away from the normal as they pass from water (denser) to air (rarer), making the pencil appear bent at the water surface.
- Trace the path of rays in the following ray diagrams:
Answer – 1. Convex Lens (Converging):
Parallel ray → Passes through focus (F).
Central ray → Goes straight through optical center (O).
Ray through F → Emerges parallel.
Result: Rays converge to form a real image (if object is beyond F).
- Concave Lens (Diverging):
Parallel ray → Appears to come from virtual focus (F).
Central ray → Undeviated.
Ray toward F → Emerges parallel.
Result: Rays diverge; forms a virtual image.
- Glass Slab:
Incident ray → Bends toward normal (air→glass).
Through slab → Straight path.
Emergent ray → Bends away from normal (glass→air).
Result: Emergent ray is parallel but displaced.
- When a spherical lens is held towards the Sun and a sharp image is formed on a piece of black paper, a hole gets burnt in the black paper, after some time.
(a) Name the lens used in the above activity.
Answer – Convex lens (converges sunlight to a point).
(b) What name is given to the distance between the spherical lens and the black paper?
Answer – Focal length (distance where sunlight converges sharply).
(c) What is the relative advantage of using a black paper rather than a white paper?
Answer – Black absorbs more heat/light, causing faster burning.
- What kind of lens would form—
(a) an inverted and magnified image of the object?
Answer – Convex lens (object between F and 2F).
(b) an erect and magnified image of the object?
Answer – Convex lens (object within focal length).
(c) an inverted and diminished image of the object?
Answer – Convex lens (object beyond 2F).
(d) an erect and diminished image of the object?
Answer – Concave lens (always forms such images).
- Draw labelled ray diagrams to illustrate the differences between the virtual images formed by using (a) a concave lens (b) a convex lens.
Answer – (a) Concave lens:
Rays diverge; virtual image forms between lens and focus (erect, diminished).
Diagram: Incident parallel rays diverge, extended backward to meet at a virtual focus.
(b) Convex lens (as magnifying glass):
Object within focal length; virtual image forms on the same side (erect, enlarged).
Diagram: Rays diverge after refraction, extended backward to form an enlarged image.
E. Answer the following questions.
- A coin is placed at the bottom of a dear glass vessel. An observer moves herself to a position where the coin goes just out of sight of her eyes. Keeping her eyes in that position, she asks her friend to pour some water, gradually into the glass vessel.
(a) What would she observe?
Answer – The observer will see the coin reappear as water is poured into the vessel.
(b) Name the phenomenon involved in this observation.
Answer – Refraction of light (light rays from the coin bend away from the normal as they pass from water to air, making the coin visible again).
(c) Draw a neat labelled diagram to justify your answer.
Answer – Labelled Diagram:
Copy
Download
Observer’s eye
↑
| (Apparent position of coin)
┌────┴────┐
│ Water │ ← Bent light rays
└────┬────┘
- (Actual coin at bottom)
Explanation: Light rays bend at the water-air interface, allowing the coin to become visible.
- In the following figures, identify the light ray that would be the correct cmergcnt/rcfractcd light ray in each case. Give reason for your answer in each ease.
Answer – Figure (a): Air to Glass
Correct Ray: The ray that bends toward the normal (closer to the perpendicular line).
Reason: When light travels from air (rarer medium) to glass (denser medium), it slows down and bends toward the normal.
Figure (b): Air to Water
Correct Ray: The ray that bends toward the normal but less sharply than in glass.
Reason: Water is less optically dense than glass, so the bending is less pronounced but still toward the normal.
Figure (c): Glass to Air
Correct Ray: The ray that bends away from the normal.
Reason: When light travels from glass (denser medium) to air (rarer medium), it speeds up and bends away from the normal.3. White light is made to fall on one face of a glass prism. Draw a ray diagram to show the nature of the emergent beam.
Answer – White light → Prism → VIBGYOR spectrum (Violet bends most, red least).
Explanation: Dispersion occurs due to different refractive indices for different colors.
- With the help of a suitable diagram, explain the meaning of terms:
(a) centres of curvature (b) optical centre (c) focus ‘
Answer – (a) Centres of Curvature (C₁, C₂):
Points at the centers of the spheres forming the lens surfaces.
(b) Optical Centre (O):
A point on the principal axis where light passes undeviated.
(c) Focus (F):
Point where parallel rays converge (convex) or appear to diverge (concave).
Diagram:
C₁ F C₂
- ——-·——-·
\ | /
\ | /
Lens
- We are given a convex lens of focal length 20 cm. Draw ray diagrams to show i the nature, size and position of the image formed when the object is kept at a , distance of
(a) 55 cm
Answer- Image: Real, inverted, diminished (between F and 2F).
(b) 35 cm
Answer- Image: Real, inverted, magnified (beyond 2F).
(c) 15 cm
Answer- Image: Virtual, erect, magnified (same side as object).
from the lens. ‘
(Note: For drawing ray diagrams, use an appropriate scale.)
- We are given a concave lens of focal length 15 cm. Draw a ray diagram to show the nature, size and position of the image formed when the object is kept at a distance of
(a) 30 cm
Answer- Image: Virtual, erect, diminished (between F and O).
(b) 15 cm
Answer- Image: Virtual, erect, diminished (closer to lens).
(c) 10 cm
Answer- mage: Virtual, erect, diminished (very close to lens).
from the lens.
(Note: For drawing ray diagrams, use an appropriate scale.)
- What type of a lens is used as a ‘magnifying glass’? How is the object positioned with respect to this lens? Draw the appropriate ray diagram.
Answer – Lens Type: Convex lens (converging).
Object Position: Placed within the focal length (u < f).
Ray Diagram:
Object (within F) → Convex Lens → Virtual, erect, magnified image (same side).
Explanation: Rays diverge after refraction; extended backward to form an enlarged image.
Value Based Questions
Khushi listened with great attention and interest when her teacher explained the focussing action of a convex lens through the ‘black-paper’ experiment. The teacher went on to say that we must have a similar focussed and dedicated approach towards the ‘task-at-hand’ to achieve success in our life.
On reaching home, Khushi shared her teacher’s ideas with her mother. Her mother fully supported her teacher’s suggestion and advised Khushi to do her studies in a focussed and concentrated way. She also advised Khushi not to get distracted by messages on social media while she was studying. Khushi promised to do so.
- Write the values depicted by (a) Khushi’s teacher (b) Khushi.
Answer – Values Depicted
(a) Khushi’s Teacher:
Inspirational Teaching: Used a science experiment to teach a life lesson.
Wisdom: Linked physics (focusing light) to life (focusing on goals).
Mentorship: Encouraged dedication and concentration.
(b) Khushi:
Attentiveness: Listened carefully in class.
Respect for Learning: Shared the lesson with her mother.
Commitment: Promised to avoid distractions and stay focused.
- Draw a diagram showing the focussing action of a convex lens in the black-paper experiment mentioned above.
Answer – Diagram: Focusing Action of a Convex Lens
Black-Paper Experiment:
Sunlight (Parallel rays)
↓
┌───────┐
│ Convex │
│ Lens │ → Focuses light to a point
└───────┘
↓
Black Paper → Burns a hole at focal point
Explanation:
The convex lens converges sunlight to a sharp focal point, concentrating heat energy to burn the paper.
- Have a group discussion with your friends, on the ‘pros and cons’ of ‘social media’ in our life.
Answer –
Pros | Cons |
1. Connectivity: Keeps us in touch with friends/family globally. | 1. Distraction: Notifications disrupt study/work focus. |
2. Learning: Access to educational content (YouTube, courses). | 2. Misinformation: Fake news spreads quickly. |
3. Awareness: Updates on global events and social issues. | 3. Addiction: Excessive use affects mental health and sleep. |
4. Opportunities: Networking for careers/business. | 4. Privacy Risks: Data leaks and cyberbullying. |
Conclusion:
Social media is a tool—its impact depends on how we use it. Balance and self-discipline (like Khushi’s resolve) are key!
Something to Do
- One of the first microscopes invented used a small drop of water as ‘magnifying glass’. The drop was held in a smooth round hole in a sheet of metal or stiff plastic. Design your own magnifying glass using a stiff plastic sheet. Examine a small object (as the text on a page) through the water drop. How does changing the size of the drop affect the magnification of the image? Record your observations.
Answer – Hands-On Experiments
- DIY Water Drop Magnifying Glass
Materials Needed:
Stiff plastic sheet (e.g., from a folder)
Small object (text on paper, ant, etc.)
Water dropper
Steps:
Cut a small, smooth hole (3–5 mm diameter) in the plastic sheet.
Place a water drop over the hole—it will bulge due to surface tension.
Hold the sheet close to the object and observe through the drop.
Observations:
Smaller drop: Higher curvature → Stronger magnification but smaller field of view.
Larger drop: Flatter → Less magnification but wider view.
Effect: The water drop acts as a convex lens, bending light to create a virtual, magnified image.
Scientific Principle:
The curved surface of the water drop refracts light rays, functioning like a plano-convex lens.
- Use two appropriate convex lenses and two cylindrical/cardboard tubes to design an optical telescope.
Answer – DIY Optical Telescope
Materials Needed:
Two convex lenses:
Objective lens (large focal length, e.g., +50 cm)
Eyepiece lens (short focal length, e.g., +5 cm)
Two cardboard tubes (one slightly wider to slide over the other)
Steps:
Fix the objective lens at one end of the wider tube.
Fix the eyepiece lens at one end of the narrower tube.
Slide the narrower tube inside the wider one to adjust focus.
How It Works:
The objective lens collects light from distant objects and forms a real, inverted image inside the tube.
The eyepiece lens magnifies this image for the eye.
Tip: Use lenses from old spectacles or magnifying glasses.
- Take a tub/trough full of water and place a light source (like a water proof pencil torch) inside it. Look at the light beam from above. Now, tilt the pencil torch t6 change the angle of incidence. Observe carefully what happens? Do you observe any refracted beam when you increase the angle of incidence beyond a certain value?
Answer – Observing Refraction and Total Internal Reflection
Materials Needed:
Transparent tub/trough filled with water
Waterproof torch (tiltable)
Dark room
Steps:
Submerge the torch and turn it on.
View the light beam from above the water surface.
Gradually tilt the torch to increase the angle of incidence.
Observations:
At small angles: Light refracts out of the water (bends away from normal).
Beyond critical angle (~48° for water):
No refracted beam is seen above water.
Light undergoes total internal reflection (appears as a bright underwater beam).
Scientific Principle:
When light travels from denser (water) to rarer (air) medium:
If the angle of incidence > critical angle, refraction stops, and 100% reflection occurs.
Real-world example: Optical fibers and mirages.
Additional Questions – 01
Short Questions & Answers
- What is refraction of light?
Ans: Refraction is the bending of light when it passes obliquely from one transparent medium to another due to a change in its speed.
- State the two laws of reflection.
Ans: The incident ray, reflected ray, and normal at the point of incidence all lie in the same plane.
The angle of incidence (∠i) is equal to the angle of reflection (∠r).
- Why does a pencil appear bent when dipped in water?
Ans: Light rays from the pencil bend away from the normal as they pass from water (denser) to air (rarer), making the pencil appear bent.
- Define refractive index.
Ans: Refractive index of a medium is the ratio of the speed of light in vacuum to its speed in that medium:
μ= c/v
5. What is the cause of refraction?
Ans: Refraction occurs due to the change in the speed of light when it moves from one medium to another.
- Why does a convex lens converge light rays?
Ans: A convex lens is thicker at the center, causing light rays to bend inward (converge) due to refraction.
- What is dispersion of light?
Ans: Dispersion is the splitting of white light into its seven constituent colors (VIBGYOR) when passed through a prism due to different refractive indices for different colours.
- What type of image is formed by a concave lens?
Ans: A concave lens always forms a virtual, erect, and diminished image.
- Why does a pond appear shallower than it actually is?
Ans: Light rays from the bottom of the pond bend away from the normal as they pass from water to air, making the pond appear shallower.
- What is the critical angle?
Ans: The critical angle is the angle of incidence in a denser medium for which the angle of refraction in the rarer medium is 90°.
Long Questions & Answers
- Explain refraction through a glass slab with a diagram.
Ans: When light enters a glass slab (denser medium) from air (rarer medium), it bends towards the normal.
Inside the slab, it travels straight.
When exiting, it bends away from the normal.
The emergent ray is parallel to the incident ray but laterally displaced.
Diagram:
Incident ray → / (bends towards normal)
Inside slab → | (straight)
Emergent ray → \ (bends away from normal)
- Describe an experiment to show the dispersion of light.
Ans: Apparatus: Glass prism, white light source, screen.
Procedure:
Allow a narrow beam of white light to pass through a prism.
Observe the light splitting into VIBGYOR colors on a screen.
Reason: Different colors have different speeds in glass, causing different bending angles.
- Differentiate between convex and concave lenses.
Ans:
Feature | Convex Lens | Concave Lens |
Shape | Thicker at the center | Thinner at the center |
Light bending | Converges light rays | Diverges light rays |
Image formed | Can be real or virtual | Always virtual, erect, and diminished |
Uses | Magnifying glass, cameras, telescopes | Correcting myopia, peepholes |
- Explain why stars twinkle but planets do not.
Ans: Stars twinkle because their light passes through Earth’s turbulent atmosphere, causing refraction at varying angles (scintillation).
Planets do not twinkle because they are closer and appear as extended sources, averaging out the refraction effects.
- What is total internal reflection? Give two applications.
Ans: Definition: When light travels from a denser to a rarer medium at an angle greater than the critical angle, it reflects entirely back into the denser medium.
Applications:
Optical fibers (used in communication).
Mirage formation in deserts.
- Draw a ray diagram for a convex lens when the object is placed between F and 2F.
Ans: Image characteristics:
Real, inverted, magnified.
Formed beyond 2F on the other side.
Diagram:
Object (between F & 2F) → Convex Lens → Real, inverted, magnified image (beyond 2F)
- Why does a diamond sparkle more than glass?
Ans: Diamond has a very high refractive index (≈2.42), causing more bending of light.
Its small critical angle (≈24.4°) promotes total internal reflection, enhancing sparkle.
- How does the human eye focus on near and far objects?
Ans: The eye lens adjusts its curvature using ciliary muscles:
Near objects: Lens becomes thicker (more converging).
Far objects: Lens becomes thinner (less converging).
Multiple Choice Questions (MCQs)
What happens to light when it passes from air into glass?
- a) Speeds up and bends toward the normal
- b) Slows down and bends toward the normal ✅
- c) Speeds up and bends away from the normal
- d) Slows down and bends away from the normal
The refractive index of diamond is 2.42. This means:
- a) Light travels 2.42× faster in diamond than in air
- b) Light travels 2.42× slower in diamond than in vacuum ✅
- c) Diamond reflects 2.42× more light than glass
- d) Diamond is 2.42× denser than water
A concave lens always forms an image that is:
- a) Real and inverted
- b) Virtual and erect ✅
- c) Real and magnified
- d) Virtual and inverted
Which color of light bends the MOST when passing through a prism?
- a) Red
- b) Green
- c) Violet ✅
- d) Yellow
Total internal reflection occurs when light travels from:
- a) Air to water
- b) Water to air ✅
- c) Glass to water
- d) Air to glass
The focal length of a convex lens is 10 cm. Where should an object be placed to get a real, inverted, and same-sized image?
- a) At 10 cm
- b) At 20 cm ✅
- c) At 5 cm
- d) At infinity
A mirage is caused by:
- a) Reflection
- b) Dispersion
- c) Total internal reflection ✅
- d) Diffraction
Which lens is used to correct myopia (nearsightedness)?
- a) Convex lens
- b) Concave lens ✅
- c) Cylindrical lens
- d) Bifocal lens
The angle of incidence for which the angle of refraction is 90° is called:
- a) Critical angle ✅
- b) Brewster’s angle
- c) Refractive angle
- d) Scattering angle
A light ray passes from glass (μ=1.5) to water (μ=1.33). It will:
- a) Bend toward the normal
- b) Bend away from the normal ✅
- c) Not bend at all
- d) Reflect completely
True/False Questions
Light travels faster in water than in glass.
True ✅ (Speed in water ≈ 2.25×10⁸ m/s; in glass ≈ 2×10⁸ m/s)
A convex lens can form both real and virtual images.
True ✅ (Real when object is beyond F; virtual when within F)
The critical angle depends on the wavelength of light.
True ✅ (Different colors have different critical angles due to dispersion)
A concave mirror can converge light rays.
True ✅ (Concave mirrors are converging mirrors)
The human eye lens is a concave lens.
False ❌ (It’s a flexible convex lens)
Dispersion occurs because light rays of different colors have different speeds in a medium.
True ✅
A magnifying glass uses a concave lens.
False ❌ (Uses a convex lens)
The focal length of a lens changes if it is submerged in water.
True ✅ (Due to change in refractive index contrast)
Rainbows are formed due to reflection of sunlight by water droplets.
False ❌ (Due to refraction + dispersion + TIR in droplets)
A convex lens of focal length 20 cm has more power than a 10 cm convex lens.
False ❌ (Power = 1/f; 10 cm lens has higher power)
Advanced MCQs
A light ray bends toward the normal when entering a medium. The new medium is:
- a) Optically rarer
- b) Optically denser ✅
- c) Same density as air
- d) A perfect vacuum
Which phenomenon explains why stars twinkle?
- a) Dispersion
- b) Atmospheric refraction ✅
- c) Total internal reflection
- d) Scattering
The power of a lens is measured in:
- a) Watts
- b) Diopters ✅
- c) Lux
- d) Lumens
A lens forms a virtual, erect, and magnified image. The object must be placed:
- a) At F
- b) Between F and 2F
- c) Beyond 2F
- d) Within F ✅
Which device uses both a convex and concave lens?
- a) Microscope
- b) Telescope ✅
- c) Periscope
- d) Kaleidoscope
Diagram-Based MCQs (Assume diagrams are provided)
Which diagram shows correct refraction from air to water?
Option where ray bends toward the normal ✅
Identify the emergent ray in a glass slab experiment:
Ray parallel to incident ray but displaced ✅
Which prism diagram shows proper dispersion of white light?
Violet deviated most, red least ✅
Bonus: Application-Based Questions
A fisherman sees a fish underwater. To spear it, he should aim:
- a) Directly at the fish
- b) Below the fish ✅ (Due to refraction)
- c) Above the fish
- d) To the side
Why are concave lenses used in movie projectors?
- a) To magnify the image
- b) To correct spherical aberration ✅
- c) To diverge light rays
- d) To reduce glare