Light travels from glass (n = 1.5) into water (n = 1.33). What happens to the light ray at the boundary?
AIt bends toward the normal, since it enters a denser medium
BIt bends away from the normal, since it enters a medium with a lower refractive index and speeds up
CIt continues straight — bending only occurs when going from lower to higher refractive index
DIt reflects entirely — light cannot pass from glass to water
When light crosses from a higher-index medium (glass, n=1.5) to a lower-index medium (water, n=1.33), it speeds up. Snell's law confirms: n₁ sin θ₁ = n₂ sin θ₂; with n₁ > n₂, sin θ₂ > sin θ₁, so the refracted angle is larger — the ray bends away from the normal. The common misconception (option A) is that 'denser always means toward the normal.' The direction depends entirely on which side has the higher index: higher to lower means away from normal, lower to higher means toward normal.
Question 2 Multiple Choice
A student reasons: 'When light slows down entering glass from air, its frequency must decrease since v = fλ.' What is wrong with this reasoning?
ANothing — frequency does decrease proportionally when the wave slows down
BFrequency is set by the source and cannot change at a boundary — wavelength decreases instead, since v = fλ and f is fixed
CThe formula v = fλ doesn't apply at boundaries, only inside a homogeneous medium
DSpeed doesn't actually change at the boundary — only the direction of propagation changes
Frequency is determined by the source — wave crests are emitted at a fixed rate and arrive at the boundary at that same rate. They cannot pile up or disappear, so frequency is invariant across any boundary. Since v = fλ and f is constant, a decrease in speed (denser medium) must be accompanied by a decrease in wavelength. This wavelength compression — not frequency change — is the mechanical cause of refraction. The explainer makes this explicit: 'a slower medium means a shorter wavelength.'
Question 3 True / False
A light ray entering a flat glass slab at an angle bends toward the normal at entry, then bends away from the normal at exit — so the emergent ray is parallel to the incident ray, only laterally displaced.
TTrue
FFalse
Answer: True
At the first surface (air→glass), the ray slows and bends toward the normal. At the second parallel surface (glass→air), it speeds up and bends away from the normal by exactly the same angle. Since both surfaces are parallel, the two refractions cancel, and the exit ray is parallel to the entry ray — just shifted sideways. The explainer states: 'A flat slab of glass produces two parallel refractions that cancel out, leaving the beam displaced but not deflected.'
Question 4 True / False
A straw appears bent in a glass of water because light travels at different speeds through the glass container versus the water.
TTrue
FFalse
Answer: False
The apparent bend is caused by refraction at the water-air interface, not the glass. Light from the submerged portion of the straw travels through water, strikes the water-air boundary, and refracts (changes direction) before reaching the eye. The glass container is not the relevant boundary for this effect — a straw in an open bowl of water with no glass would appear equally bent. The interface between two media with different refractive indices is what causes bending, and here that interface is water-to-air.
Question 5 Short Answer
When a wave crosses a boundary from one medium into another, why does its wavelength change but not its frequency?
Think about your answer, then reveal below.
Model answer: Frequency is determined by the source — the number of wave crests generated per second is fixed before the wave reaches the boundary. At the interface, crests arrive at exactly the rate they were emitted and depart at the same rate; they cannot accumulate or disappear. So frequency is conserved across any boundary. Since the wave speed changes (due to the new medium's physical properties) and v = fλ with f fixed, wavelength must change to compensate: λ = v/f. A slower medium means shorter wavelength.
Frequency invariance at boundaries is a universal property of wave behavior, not specific to light. If frequency changed, wave crests would either pile up (building infinite amplitude) or thin out at the interface — physically impossible in steady state. The angular bending of Snell's law is the geometric consequence of wavefront portions with different wavelengths arriving at the boundary simultaneously.