NCERT Solutions Class 8 Science Chapter 10 Light Mirrors and Lenses

 

Question 1. A light ray is incident on a mirror and gets reflected by it (Fig. 10.21). The angle made by the incident ray with the normal to the mirror is 40°. What is the angle made by the reflected ray with the mirror?
(i) 40°
(ii) 50°
(iii) 45°
(iv) 60°
Answer: (ii) 50°
In simple words: When light hits a mirror at 40° from the straight line (normal), it bounces back at the same angle - also 40°. Since the mirror itself is at 90° to the normal, the angle between the bounced light and the mirror surface becomes 90° - 40° = 50°.

Exam Tip: Remember the law of reflection: angle of incidence always equals angle of reflection, both measured from the normal, not the mirror surface. The question asks for the angle with the mirror, so subtract from 90°.

 

Question 2. Fig. 10.22 shows three different situations where a light ray falls on a mirror: (i) The light ray falls along the normal. (ii) The mirror is tilted, but the light ray still falls along the normal to the tilted surface. (iii) The mirror is tilted and the light ray falls at an angle of 20° from the normal. Draw the reflected ray in each case (Use a ruler and protractor for accurate drawing). What is the angle of reflection in each case?
Answer:
For all three cases, the angle of reflection must equal the angle of incidence, as stated by the law of reflection:
(i) When the light ray falls along the normal: angle of incidence = 0°, so angle of reflection = 0°. The ray bounces straight back along the same path.
(ii) When the mirror is tilted but the ray still hits the normal of the tilted surface: angle of incidence = 0°, so angle of reflection = 0°. The ray bounces back along the normal of the tilted mirror.
(iii) When the mirror is tilted and the ray hits at 20° from the normal: angle of incidence = 20°, so angle of reflection = 20°. The reflected ray makes a 20° angle on the opposite side of the normal.
In simple words: In every situation, the bounce angle equals the hitting angle, both measured from the straight line perpendicular to the mirror. This rule works no matter how the mirror is tilted or turned.

Exam Tip: Always draw the normal line first (perpendicular to the mirror surface), then measure both angles from this normal line. The reflected ray should be on the opposite side of the normal from the incident ray.

 

Question 3. In Fig. 10.23, the cap of a sketch pen is placed in front of three types of mirrors.
Answer: A plane mirror shows an image that is upright and the same size as the object. A concave mirror produces images that vary depending on where the object sits - the image may be enlarged and upright when close, or enlarged and upside-down when farther away. A convex mirror always creates an upright image that is smaller than the object, no matter the distance.
In simple words: A flat mirror gives you a normal reflection. A curved-inward mirror can make things look bigger or upside-down. A curved-outward mirror always makes things look smaller.

Exam Tip: Learn the image characteristics for each mirror type - flat mirrors are predictable, but curved mirrors require you to know both the mirror shape and the object distance to describe the image correctly.

 

Question 4. In Fig. 10.24 the cap of a sketch pen is placed behind a convex lens, a concave lens and a flat transparent glass piece - all at the same distance. Match each image with the correct type of lens or glass.
Answer:
(i) Flat transparent glass piece - Image (iii): The object appears in its normal position and size, with no magnification or reduction. Flat glass does not bend light significantly, so the image stays upright and unchanged.
(ii) Convex lens - Image (i): The object appears magnified and upright. A convex lens brings light rays together (converges them), making objects behind it look larger when viewed from the front.
(iii) Concave lens - Image (ii): The object appears smaller than its actual size. A concave lens spreads light rays apart (diverges them), making objects look reduced and farther away.
In simple words: Flat glass keeps things looking normal. A curved-outward lens makes things look bigger. A curved-inward lens makes things look smaller.

Exam Tip: Focus on whether the lens curves outward (convex - makes bigger) or inward (concave - makes smaller). Flat glass never changes the size of the image, only its position slightly.

 

Question 5. When the light is incident along the normal on the mirror, which of the following statements is true:
(i) Angle of incidence is 90°
(ii) Angle of incidence is 0°
(iii) Angle of reflection is 90°
(iv) No reflection of light takes place in this case
Answer: (ii) Angle of incidence is 0°
In simple words: The normal is a line perpendicular (standing straight up) from the mirror surface. When light travels along this normal, it hits the mirror head-on with zero angle between the light ray and the normal. The light bounces straight back the way it came - this is called the ray retracing its path.

Exam Tip: When a question says "light is incident along the normal," remember that angle of incidence is ZERO by definition. The ray bounces directly backward, so angle of reflection is also zero.

 

Question 6. Three mirrors - plane, concave and convex are placed in Fig. 10.25. On the basis of the images of the graph sheet formed in the mirrors, identify the mirrors and write their names above the mirrors.
Answer: Looking at the images formed by each mirror:
- Convex mirror (left): Forms an upright, smaller image. The grid squares appear reduced in size, and the image is right-side up.
- Plane mirror (center): Forms an upright image of the same size as the object. The grid squares match the original size and spacing, appearing as a normal mirror reflection.
- Concave mirror (right): Forms an image that depends on distance. In this case, the grid appears enlarged or may be inverted if the object is beyond the center of curvature.
In simple words: A bulging-outward mirror makes things look tiny. A flat mirror shows them normal size. A curved-inward mirror makes them look big or flips them upside-down.

Exam Tip: Use the image size and orientation as clues. If the image is always smaller and upright, it is convex. If the image is the same size and upright, it is plane. If the image changes size or gets inverted, it is concave.

 

Question 7. In a museum, a woman walks towards a large convex mirror (Fig. 10.26). She will see that:
(i) her erect image keeps decreasing in size.
(ii) her inverted image keeps decreasing in size.
(iii) her inverted image keeps increasing in size and eventually it becomes erect and magnified.
(iv) her erect image keeps increasing in size.
Answer: (iii) her inverted image keeps increasing in size and eventually it becomes erect and magnified.
In simple words: As a person moves closer to a concave mirror, the mirror's power to bend light becomes stronger. The image gets bigger and bigger. At very close distances, the image flips right-side up and looks much larger than the person - this is why concave mirrors are used in makeup mirrors and shaving mirrors.

Exam Tip: Remember that as an object moves closer to a concave mirror, the image transitions from inverted and small (far away) to inverted and large (middle distance) to erect and magnified (very close). This change happens because the object crosses the center of curvature, then the focal point.

 

Question 8. Hold a magnifying glass over text and identify the distance where you can see the text bigger than they are written. Now move it away from the text. What do you notice? Which type of lens is a magnifying glass?
Answer: When you hold a magnifying glass close to text, the letters appear much larger and clearer - this is the magnified view. As you move the magnifying glass away from the text, the magnification decreases and the letters start to shrink. If you move it even farther, the text may become inverted and upside-down, or it may shrink to a very small size. A magnifying glass is a convex lens - it curves outward on both sides (or at least on one side), and it collects light rays to focus them, making objects appear larger when held at the correct distance from them.
In simple words: When you hold a magnifying glass close to words, they look huge. As you pull it away, they get smaller and eventually flip upside-down. A magnifying glass is a curved-outward lens that bends light to make things look bigger.

Exam Tip: The focal length is the key distance. Holding the lens closer than the focal length gives magnification; moving beyond it causes the image to invert and shrink. Always test a magnifying glass by adjusting it slowly to find the magnified-view distance.

 

Question 9. Match the entries in Column I with those in Column II.

Answer:
The correct matches are:
(i) Concave mirror - (a) Spherical mirror with a reflecting surface that curves inwards.
(ii) Convex mirror - (b) It forms an image which is always erect and diminished in size.
(iii) Convex lens - (c) Object placed behind it may appear inverted at some distance.
(iv) Concave lens - (d) Object placed behind it always appears diminished in size.
In simple words: A curved-inward mirror (concave) has its surface pushing in. A curved-outward mirror (convex) always shrinks what you see. A curved-outward lens (convex) can flip things upside-down if they are far enough. A curved-inward lens (concave) always makes things look smaller and farther.

Exam Tip: Learn the defining properties: concave mirror can invert, convex mirror always shrinks, convex lens can magnify or invert depending on distance, concave lens always reduces size. These properties help you match correctly even without seeing a figure.

 

Question 10. Assertion: Convex mirrors are preferred for observing the traffic behind us. Reason: Convex mirrors provide a significantly larger view area than plane mirrors. Choose the correct option:
(i) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
(ii) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
(iii) Assertion is correct but Reason is incorrect.
(iv) Both Assertion and Reason are incorrect.
Answer: (i) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
In simple words: Convex mirrors are used in cars and stores because they let you see a wide area behind or around you, even though objects look smaller. This wide viewing area is exactly why they are chosen over flat mirrors.

Exam Tip: In assertion-reason questions, check both statements for correctness first, then decide if the reason actually explains the assertion. Here, both are true, and the wide field of view (reason) is indeed why convex mirrors are used (assertion).

 

Question 11. In Fig. 10.27, note that O stands for object, M for mirror, and I for image. Which of the following statements is true?
(i) Figure (a) indicates a plane mirror and Figure (b) indicates a concave mirror.
(ii) Figure (a) indicates a convex mirror and Figure (b) indicates a concave mirror.
(iii) Figure (a) indicates a concave mirror and Figure (b) indicates a convex mirror.
(iv) Figure (a) indicates a plane mirror and Figure (b) indicates a convex mirror.
Answer: (iv) Figure (a) indicates a plane mirror and Figure (b) indicates a convex mirror.
In simple words: In figure (a), the object and image are at the same distance from the mirror and are the same size - this is how a flat mirror works. In figure (b), the image is closer to the mirror than the object, showing the curved-outward behavior of a convex mirror that shrinks and moves images closer.

Exam Tip: Use image position and size to identify mirrors. Plane mirrors always place the image behind the mirror at the same distance and size as the object. Convex mirrors always place the image between the mirror and the object, reduced in size.

 

Question 12. Place a pencil behind a transparent glass tumbler (Fig. 10.28a). Now fill the tumbler halfway with water (Fig. 10.28b). How does the pencil appear when viewed through the water? Explain why its shape appears changed.
Answer: When you look at the pencil through the water, it appears bent or displaced at the water surface, even though the pencil itself is straight. The part of the pencil that is in the water seems to be shifted or broken at the boundary where water meets air. This happens because of refraction - when light travels from one transparent material to another (like from water to air), it bends at the boundary. Light bends differently depending on the angle at which it hits the surface and the materials involved. Since water bends light more than air does, the light rays coming from the pencil in the water bend as they leave the water and enter the air above it. This bending makes our eyes see the pencil in a different position than it actually is, creating the illusion of a bent or broken pencil.
In simple words: Light bends when it moves from water into air. This bending tricks your eye into thinking the pencil is bent, even though it is actually straight. The bending is called refraction.

Exam Tip: Refraction always occurs when light crosses a boundary between two transparent materials with different densities. Remember that the ray bends toward the normal when moving from a less dense to a denser medium, and away from the normal when moving from denser to less dense. Water is denser than air, so light bends away from the normal when leaving the water.