Mirage is an illustration of
- (a)only dispersion of light.
- (b)only reflection of light.
- (c)only total internal reflection of light.
- (d)both refraction and total internal reflection of light.
Correct — D, both refraction and total internal reflection of light. On a hot day the air near the ground is warmer and optically rarer than the cooler, denser air higher up. Light from the sky travelling downward passes through these layers of steadily decreasing density and is continuously refracted, bending more and more; where it meets the hottest, rarest layer near the ground beyond the critical angle it undergoes total internal reflection and curves back upward. The eye traces this bent ray back in a straight line and sees an inverted, shimmering image (often looking like water) — so a mirage involves both refraction and total internal reflection.
- (a)only dispersion of light. — Dispersion splits white light into its colours (as in a rainbow); a mirage is not a splitting of colours, so dispersion is not the cause.
- (b)only reflection of light. — Ordinary reflection from a surface (like a mirror) is not what happens; the light is bent gradually through air layers and then totally internally reflected, which is different from simple reflection.
- (c)only total internal reflection of light. — Total internal reflection is part of the story, but it cannot occur without the gradual refraction through the warm air layers that first bends the ray up to the critical angle — so 'only TIR' is incomplete.
A mirage is an atmospheric-optics phenomenon caused by a vertical gradient of air temperature (and hence refractive index) near a hot surface. Light bends continuously (refraction) as it crosses layers of changing density and finally undergoes total internal reflection in the hottest layer, producing a displaced, often inverted image.
The single-word 'only …' options are traps; a mirage needs both refraction (through the density gradient) and total internal reflection together. Linking 'the hot road looks wet' to refraction-plus-TIR is a reliable memory hook.
- Hot air near the ground is rarer (lower refractive index) than the cooler air above.
- Light is refracted progressively as it passes through these air layers.
- It undergoes total internal reflection at the hot rarer layer and curves upward.
- The result is an inverted, water-like image — a mirage, an example of atmospheric refraction.
- Choosing 'only total internal reflection' — refraction through the air gradient is also essential.
- Confusing a mirage with dispersion (colour splitting) or ordinary mirror reflection.
Asked as 'a mirage is due to …' or by grouping it with other atmospheric-refraction phenomena.
Total internal reflection can take place when light travels from
- (a) diamond to glass
- (b) water to glass
- (c) air to water
- (d) air to glass
Answer(a) diamond to glass
Same concept — total internal reflection, which occurs when light goes from a denser to a rarer medium beyond the critical angle (diamond → glass); the same TIR that, with refraction, produces a mirage.
Rainbow is produced when sunlight falls on drops of rain. Which of the following physical phenomena are responsible for this? 1. Dispersion 2. Refraction 3. Internal reflection. Select the correct answer using the codes given below.
- (a) 1 and 2 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Answer(d) 1, 2 and 3
Related — a rainbow, like a mirage, combines refraction with internal reflection of light, illustrating how these optical effects act together to form an image.
Light rays move in straight lines. But through an optical fibre, they can move in any type of zigzag path because
- (a) the holes through the fibre are extremely fine.
- (b) light rays are absorbed at the entry end and relieved at the exit end of the fibre.
- (c) scattering of light occurs inside the fibre.
- (d) successive total internal reflections occur as a ray moves through the fibre.
Answer(d) successive total internal reflections occur as a ray moves through the fibre.
Same concept — light guided by successive total internal reflections, the same TIR mechanism that (with refraction) turns a mirage's rays back towards the eye.
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
Related — primary-rainbow formation involves refraction plus one internal reflection of sunlight, the refraction-plus-reflection combination also behind a mirage.
- practice — not a real PYQ
On a hot day the shimmering 'water' seen on a road is a mirage. It is primarily caused by
- (a)dispersion of light
- (b)atmospheric refraction with total internal reflection
- (c)reflection from the road surface
- (d)diffraction of light
Answer(b) atmospheric refraction with total internal reflection — through hot air layers of varying density.
- practice — not a real PYQ
Total internal reflection can occur only when light travels from
- (a)a rarer to a denser medium
- (b)a denser to a rarer medium at an angle above the critical angle
- (c)any two media at 0° incidence
- (d)vacuum into glass
Answer(b) a denser to a rarer medium at an angle above the critical angle.