Bottom of a tank containing water appears to be raised. It is due to
- (a)Reflection of light
- (b)Refraction of light
- (c)Interference of light
- (d)Diffraction of light
Correct — B, refraction of light. Light rays travelling from the bottom of the tank bend away from the normal as they leave the denser water and enter the less dense air. To the eye these bent rays seem to come from a point higher than the actual bottom, so the bottom looks raised. The apparent depth is less than the real depth by a factor equal to the refractive index of water.
- (a)Reflection of light — Reflection is light bouncing off a surface, as in a mirror; it does not shift the apparent position of a submerged bottom. The raising is caused by light bending as it changes medium, which is refraction.
- (c)Interference of light — Interference is the superposition of two coherent light waves producing bright and dark fringes; it does not explain a shifted apparent depth.
- (d)Diffraction of light — Diffraction is the bending of light around edges or through very narrow openings; it is not responsible for the raised appearance of the tank bottom.
Refraction is the bending of light when it passes between media of different optical density. Because of it, objects seen under water appear shallower than they really are: the apparent depth equals the real depth divided by the refractive index of the medium.
The same effect makes a pencil look bent at the water surface, a coin in a bowl seem to rise, and a swimming pool look shallower than it is. The key insight is that light bends (refracts) at the water-air boundary — it is not a reflection.
- Refraction is the bending of light as it passes from one medium into another of different optical density.
- A tank or pool bottom appears raised because light bends away from the normal on leaving the water.
- Apparent depth equals real depth divided by the refractive index (for water, about 1.33).
- The same refraction makes a straight stick or pencil look bent at the water surface.
- Choosing reflection instead of refraction — reflection is a mirror-bounce, while this is light bending across media.
- Thinking the bottom actually rises, when only its apparent position shifts.
Asked as the raised tank or pool bottom, the bent pencil, or the coin that seems to rise — all are apparent-depth effects of refraction.
Assertion (A): A stick is dipped in water in a slanting position. If observed sideways, the stick appears short and bent at the surface of water. Reason (R): The light coming from the stick undergoes scattering from water molecules giving the stick a short and bent appearance.
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually true but R is NOT a correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer(c) A is true but R is false
Same phenomenon and the same trap. There a slanting stick looks short and bent at the surface (true), but the reason wrongly blames scattering when it is refraction; here the tank bottom appears raised for exactly the same reason — light bending at the water-air boundary.
Which of the following statements with regard to the phenomenon of the primary rainbow formation by water droplets is/are correct? 1. It involves refraction and one internal reflection of sunlight. 2. It involves refraction of sunlight only. 3. It is formed as the inner bow. 4. It may involve more than one internal reflection as well as refraction of sunlight. Select the answer using the code given below:
- (a) 1 only
- (b) 1 and 3
- (c) 3 and 4
- (d) 2 and 3
Answer(b) 1 and 3
Same core physics — refraction of light as it crosses the air-water boundary. There refraction (with one internal reflection) inside droplets forms the primary rainbow; here refraction at the water surface makes the tank bottom appear raised.
- practice — not a real PYQ
A coin lying at the bottom of a beaker of water appears to be raised. The phenomenon responsible is
- (a)reflection
- (b)refraction
- (c)dispersion
- (d)scattering
Answer(b) refraction — light bends on leaving the water, so the coin appears nearer the surface.
- practice — not a real PYQ
The apparent depth of a pond compared with its real depth is
- (a)greater than the real depth
- (b)equal to the real depth
- (c)less than the real depth
- (d)zero
Answer(c) less than the real depth — apparent depth equals real depth divided by the refractive index.