Name the phenomenon where an opaque object on the path of light becomes very small and where light has a tendency to bend around it and not walk in a straight line.
- (a)Reflection of light
- (b)Angle of incidence
- (c)Diffraction of light
- (d)Angle of refraction
Answer
Why
Correct — C. Diffraction is the bending of light around the edge of an obstacle or through a narrow slit, and it becomes visible only when the obstacle shrinks towards the size of the light's wavelength — the 'becomes very small' condition in the stem.
Light travels in straight lines while the barrier is large. Shrink it far enough and the wave spreads into the geometrical shadow instead, which is option (c).
Why the others are wrong
- (a)Reflection of light — Reflection is light bouncing back off a surface at an equal angle. It turns light away from the obstacle rather than bending it around the edge.
- (b)Angle of incidence — The angle of incidence is a measurement, the angle between an incoming ray and the normal. The stem asks for a phenomenon, and an angle is not one.
- (d)Angle of refraction — The angle of refraction is likewise an angle, not an effect. Refraction itself needs a boundary between two media, not a small obstacle in the path.
Concept
Light behaves as a wave, and every wave diffracts — it spreads out when it meets an aperture or an obstruction of a size approaching its own wavelength.
Visible light has a wavelength of a few hundred nanometres, which is why ordinary objects cast sharp shadows and diffraction stays hidden in daily life. Sound, with wavelengths near a metre, diffracts around a doorway easily — so you hear round a corner but do not see round it.
Keep the three apart by what each one needs. Refraction needs a change of medium, reflection needs a surface, diffraction needs an edge or slit.
Key facts
- Diffraction is the bending or spreading of a wave around an obstacle or through an aperture comparable to its wavelength.
- Visible light has wavelengths of a few hundred nanometres, so diffraction of light needs very small obstacles or slits.
- Refraction is the bending of light at the boundary between two media and is governed by the refractive index.
Study next
Common traps
- Choosing refraction because both words mean bending. Refraction bends light at a medium boundary, diffraction bends it around an edge.
- Picking an option that names an angle. Two of the four choices here are measurements rather than phenomena.
SSC keeps optics to plain definitions and to the person who first stated a law.
Refraction is asked from the history side on 23 Sep 2024, 12:30, GA Q.4, which wants the scientist who first stated that every substance has its own refractive index.
Related PYQs
No directly related past PYQ was found.