Light (Case Study) (Class 10 Science)

This set covers CBSE-style case-based questions on refraction and reflection of light. Students analyze real-world scenarios involving mirrors, lenses, and

Light (Case Study) (Class 10 Science)

This set covers CBSE-style case-based questions on refraction and reflection of light. Students analyze real-world scenarios involving mirrors, lenses, and

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TL;DR: This set covers CBSE-style case-based questions on refraction and reflection of light. Students analyze real-world scenarios involving mirrors, lenses…

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This set covers CBSE-style case-based questions on refraction and reflection of light. Students analyze real-world scenarios involving mirrors, lenses, and

Light (Case Study) MCQs with Answers & Explanations

Q1. A doctor uses a concave mirror to examine a patient's teeth. The mirror produces a magnified virtual image. What is the object distance if the focal length is 2 cm?

  1. Exactly 2 cm
  2. 1 cm
  3. Between 0 and 2 cm ✓ (correct)
  4. Greater than 2 cm

Explanation: For a concave mirror to produce a magnified virtual image, the object must be placed between the pole and focal length (0 < u < f). When u < f, the mirror acts like a magnifying glass.

Q2. A student shines a laser pointer at an angle of 30 degrees to the normal on a glass surface (refractive index 1.5). The light refracts into the glass. What is the approximate angle of refraction?

  1. 45 degrees
  2. 15 degrees
  3. 30 degrees
  4. 19 degrees ✓ (correct)

Explanation: Using Snell's law: n1 sin(i) = n2 sin(r). So 1 * sin(30) = 1.5 * sin(r), giving sin(r) = 0.333, r = 19.47 degrees approximately.

Q3. A photographer wants to capture a wide field of view using a convex lens. Which property of the convex lens allows this?

  1. It converges light rays ✓ (correct)
  2. It has infinite focal length
  3. It produces only virtual images
  4. It diverges light rays

Explanation: Convex lenses converge light rays and are used in cameras and telescopes to capture images over a wide angle depending on focal length.

Q4. When white light passes through a prism, it splits into colors. Which color bends the least when exiting the prism?

  1. Blue
  2. Red ✓ (correct)
  3. Green
  4. Violet

Explanation: Red light has the longest wavelength and bends the least (lowest refractive index). Violet bends the most. This is why rainbows show red on the outer edge.

Q5. A swimming pool appears shallower than it actually is due to refraction. At what angle of incidence does total internal reflection occur for light traveling from water (n=1.33) to air?

  1. 30 degrees
  2. 42 degrees
  3. 49 degrees ✓ (correct)
  4. 60 degrees

Explanation: Critical angle: sin(c) = n2/n1 = 1/1.33. c = arcsin(0.752) = 48.75 degrees, approximately 49 degrees.

Q6. A convex mirror is used as a rear-view mirror in vehicles. Why is a convex mirror preferred over a plane mirror?

  1. It reduces light intensity
  2. It produces real images
  3. It magnifies distant objects
  4. It provides a wider field of view ✓ (correct)

Explanation: Convex mirrors diverge light and produce diminished virtual images, but cover a much wider field of view compared to a plane mirror of the same size.

Q7. A lens maker's formula relates focal length to radii of curvature. If both surfaces of a symmetric convex lens have radius 10 cm and the refractive index is 1.5, what is the focal length?

  1. 10 cm ✓ (correct)
  2. 15 cm
  3. 5 cm
  4. 20 cm

Explanation: For a symmetric convex lens R1 = +10 cm and R2 = -10 cm. 1/f = (n-1)(1/R1 - 1/R2) = (1.5-1) x (1/10 + 1/10) = 0.5 x 0.2 = 0.1, so f = 10 cm.

Q8. A concave mirror with focal length 15 cm forms a real, inverted image at the center of curvature. Where is the object placed?

  1. At the focal point
  2. At the center of curvature ✓ (correct)
  3. Beyond C
  4. Between F and C

Explanation: When an object is placed at the center of curvature (u = R = 2f), a real, inverted image of the same size is formed at the center of curvature.

Case Study: The rear-view mirror

Every car carries a rear-view mirror on the driver's side, and it is never a plane mirror. The surface bulges outward, and printed along its lower edge is the warning: objects in mirror are closer than they appear. Whatever the distance of the vehicle behind — a lorry a few metres back or a motorcycle far down the road — the driver always sees it the right way up and smaller than it really is, and can take in several lanes of traffic at once in a mirror no bigger than a postcard.

Q9. Which type of mirror is used as the rear-view mirror described in the passage, and why?

  1. Plane, because it shows the true size of every vehicle
  2. Concave, because it magnifies distant vehicles
  3. Convex, because it always gives an erect image and a wide field of view ✓ (correct)
  4. Concave, because it gives a real image the driver can see

Explanation: A convex mirror always forms a virtual, erect and diminished image whatever the object distance, and because it diverges light it covers a much wider field of view than a plane mirror of the same size. Both properties are what a driver needs.

Q10. The warning printed on the mirror says objects are closer than they appear. What causes this impression?

  1. The image is inverted, confusing the driver
  2. The mirror magnifies the vehicle
  3. The mirror bends light so the vehicle really is closer than it looks
  4. The image is diminished, so the brain judges the vehicle to be further away ✓ (correct)

Explanation: A convex mirror always produces a diminished image. We normally judge distance partly from apparent size, so a smaller-looking vehicle is read as a more distant one. The warning exists because the diminution, not the distance, is what changed.

Q11. Where is the image formed by the rear-view mirror located?

  1. Behind the mirror, between the pole and the focus ✓ (correct)
  2. In front of the mirror, and it can be caught on a screen
  3. At the centre of curvature
  4. At infinity

Explanation: For a convex mirror the image is always virtual and lies behind the mirror between the pole and the principal focus, however far away the object is. Being virtual, it cannot be caught on a screen.

Q12. If a car is 10 m behind a convex mirror of focal length 2 m, the image formed will be:

  1. Real, inverted and magnified
  2. Virtual, erect and diminished ✓ (correct)
  3. Real, erect and the same size
  4. Virtual, inverted and magnified

Explanation: A convex mirror gives a virtual, erect, diminished image for every object position — the numbers only change how small the image is, never its nature. This unchanging behaviour is exactly why it is chosen for a rear-view mirror.

Case Study: The swimming pool that is deeper than it looks

A swimming pool is filled to a uniform depth of 3 metres. Standing at the edge and looking straight down, a student judges the tiled floor to be roughly 2.25 metres below the surface, and a coin lying on the bottom seems to float above the tiles. A stick held at an angle in the water appears bent at the point where it enters. The refractive index of water is about 1.33, and light travels more slowly in water than in air.

Q13. Why does the pool in the passage appear shallower than it really is?

  1. Water absorbs some of the light
  2. The swimming pool tiles reflect light upward
  3. Light slows down and bends away from the normal on leaving water, so rays appear to come from a higher point ✓ (correct)
  4. Light travels in a curve inside water

Explanation: Light leaving the water enters a rarer medium and bends away from the normal. Tracing those emerging rays back makes them appear to come from a point higher than the true floor, so the observer sees a raised, shallower bottom.

Q14. The bending of light described in the passage is called:

  1. Reflection
  2. Scattering
  3. Dispersion
  4. Refraction ✓ (correct)

Explanation: The change in direction of light as it passes from one transparent medium into another of different optical density is refraction. It happens because the speed of light differs in the two media.

Q15. For light passing from water into air, which statement is correct?

  1. It bends away from the normal because air is optically rarer ✓ (correct)
  2. It bends towards the normal because air is denser
  3. It does not bend at all
  4. It always undergoes total internal reflection

Explanation: Air is optically rarer than water, so light speeds up on entering it and bends away from the normal. Total internal reflection happens only beyond the critical angle, not for every ray.

Q16. The refractive index of water is about 1.33. What does this number mean?

  1. Light travels 1.33 times faster in water than in vacuum
  2. Light travels about 1.33 times faster in vacuum than in water ✓ (correct)
  3. Water is 1.33 times denser than air by mass
  4. The pool appears 1.33 times deeper than it is

Explanation: Refractive index is the ratio of the speed of light in vacuum to its speed in the medium. A value of 1.33 means light travels about 1.33 times faster in vacuum than in water. It is an optical ratio, not a statement about mass density.

Case Study: The dentist's mirror

A dentist examining a patient's tooth uses a small mirror with an inward-curving reflecting surface, mounted on a long handle. Holding it very close to the tooth, the dentist sees an enlarged, upright view of the surface being examined. The same mirror is also angled to throw the light from the overhead lamp onto the tooth, brightening the area under examination. The mirror's radius of curvature is 4 cm.

Q17. Why does the dentist in the passage hold the mirror closer to the tooth than its focal length?

  1. Because a concave mirror only works at close range
  2. To reduce the brightness of the reflected light
  3. To obtain an enlarged, erect, virtual image of the tooth ✓ (correct)
  4. To obtain a real, inverted image on a screen

Explanation: A concave mirror gives an enlarged, erect, virtual image only when the object lies between the pole and the focus. Placing the tooth closer than the focal length is therefore what produces the magnified upright view the dentist needs.

Q18. The same dentist's mirror is also used to concentrate light on the tooth. Which property allows this?

  1. A concave mirror disperses white light into colours
  2. A concave mirror absorbs stray light
  3. A concave mirror diverges light over a wide area
  4. A concave mirror converges the light falling on it ✓ (correct)

Explanation: A concave mirror is a converging mirror: parallel rays striking it are brought together at the principal focus. That is why the same shape is used in torches, headlamps and solar cookers to gather or direct light.

Q19. If the tooth were instead placed beyond the centre of curvature of the mirror, the image would be:

  1. Real, inverted and diminished ✓ (correct)
  2. Real, erect and magnified
  3. Virtual, inverted and diminished
  4. Virtual, erect and magnified

Explanation: For a concave mirror with the object beyond C, the image forms between F and C and is real, inverted and diminished — useless for examining a tooth, which is precisely why the mirror is held close instead.

Q20. A concave mirror of radius of curvature 4 cm is used. Its focal length is:

  1. 4 cm
  2. 2 cm ✓ (correct)
  3. 8 cm
  4. 1 cm

Explanation: For a spherical mirror the focal length is half the radius of curvature, f = R/2, so f = 4/2 = 2 cm. The tooth must then sit within 2 cm of the mirror for the magnified erect image.

Frequently Asked Questions

What is the difference between real and virtual images formed by mirrors?

Real images are formed by actual convergence of light rays and can be projected on a screen. Virtual images are formed by divergence or apparent intersection of light rays and cannot be projected. Mirrors form real images when the object is beyond the focal point (concave) and virtual images when the object is between the pole and focal point.

Why do we use Snell's law only when light travels between two different media?

Snell's law describes how light bends when entering a medium with a different refractive index. Within a homogeneous medium, light travels in straight lines without bending. The law relates the angles of incidence and refraction to the refractive indices of the two media using n1 sin(theta1) = n2 sin(theta2).

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