به انگلیسی

Bending light with glass

Science · Waves, Light & Sound · ages 15-16

Name ______________________ Date ____________

Light of a single frequency crosses a stationary boundary from air into water. Which quantities change?
- Speed and wavelength
- Frequency only
- Speed, frequency and wavelength
You view a submerged straw through the water's surface from air at a slant. Why can it look displaced from the part above water?
- Water changes the straw's colour and physically curves its shape
- Light from underwater bends as it speeds up into air
- The glass enlarges every part equally and curves the whole straw
A ray travels along the normal from air into ordinary glass. Its speed decreases. What happens to its direction?
- It turns towards the normal despite already travelling along it
- It turns away from the normal because its speed decreases
- It continues in the same direction
How does a camera focus a distant scene using its converging lens?
- A virtual image from an object inside F lands on the sensor
- Every scene is magnified without actual rays meeting at the sensor
- A real, inverted image of the distant scene lands on the sensor
A stamp is inside the focal length of an ideal thin converging lens. What image does an observer on the outgoing side see?
- Real, inverted and reduced
- Virtual, upright and magnified
- Real, inverted and magnified
Optical fibres can carry light signals using total internal reflection.
Circle one: True False
For a stamp inside a converging lens's focal length, outgoing rays spread apart and their backward extensions meet on the stamp's side. Why can a screen there not display this image?
- Only extensions meet there; actual outgoing rays do not
- Actual rays meet there but move too slowly to reach a screen
- Actual rays meet there but the screen must be on the other side
A student draws a transmitted ray entering ordinary glass from air at nonzero incidence, bending away from the normal. What corrects the drawing?
- Its speed increases, so the away-from-normal direction is correct
- It should bend towards the normal as its speed decreases
- Its speed stays fixed, so the ray should continue without bending

Answer key

For grown-ups. Fold this page away before handing over the rest.

Bending light with glass W1-mt_u8GKuBMmzC-s1

Speed and wavelength · Frequency stays fixed. Speed decreases in water, and v = fλ gives a shorter wavelength.
Light from underwater bends as it speeds up into air · Light from the submerged straw refracts out of water. Tracing its final direction backward shifts the apparent position.
It continues in the same direction · At normal incidence, speed can change without a direction change.
A real, inverted image of the distant scene lands on the sensor · The distant scene is beyond 2F. Actual rays form a real, inverted, reduced image on the sensor.
Virtual, upright and magnified · Backward extensions locate an upright, magnified virtual image on the stamp's side.
True · At a higher-index core to lower-index cladding boundary, rays above the critical angle reflect internally along the fibre.
Only extensions meet there; actual outgoing rays do not · The apparent image location is found by backward extensions. Actual outgoing rays do not converge there to form a screen image.
It should bend towards the normal as its speed decreases · Ordinary glass slows light relative to air. At nonzero incidence, the transmitted ray bends towards the normal.