QuestionAn angler standing on a river bank finds it hard to see fish in the water because of glare from the water surface. She puts on Polaroid sunglasses and can now see the fish clearly.(a) Explain why the sunglasses reduce the glare. (3)
(b) She tilts her head sideways through 90°. State and explain what happens to the glare. (2)
(c) Suggest how she could check that the reflected light is polarised, using only her sunglasses. (1)Answer(a) Light reflected from the water surface is (partially) plane polarised. (1) The reflected light is mostly polarised horizontally, parallel to the water surface. (1) The sunglasses have a vertical transmission axis, so they absorb most of the horizontally polarised reflected light, while light from the fish, which is not mainly horizontally polarised, still passes through. (1)(b) The glare increases (returns). (1) The transmission axis of the lenses is now horizontal, so the horizontally polarised reflected light is transmitted. (1)(c) Hold the sunglasses in front of one eye, look at the reflection and rotate them: the glare changes from brightest to dimmest every 90°. (1)Examiner tips- Say that the reflected light is polarised. It is the reflection, not the sunglasses, that first polarises the glare.
- State the direction of polarisation (horizontal) and the direction of the transmission axis (vertical). Vague answers such as "the glasses block the polarised light" lose marks.
- Reflected light is completely polarised only at the Brewster angle. At other angles it is partially polarised, so some glare remains.
(b) She tilts her head sideways through 90°. State and explain what happens to the glare. (2)
(c) Suggest how she could check that the reflected light is polarised, using only her sunglasses. (1)Answer(a) Light reflected from the water surface is (partially) plane polarised. (1) The reflected light is mostly polarised horizontally, parallel to the water surface. (1) The sunglasses have a vertical transmission axis, so they absorb most of the horizontally polarised reflected light, while light from the fish, which is not mainly horizontally polarised, still passes through. (1)(b) The glare increases (returns). (1) The transmission axis of the lenses is now horizontal, so the horizontally polarised reflected light is transmitted. (1)(c) Hold the sunglasses in front of one eye, look at the reflection and rotate them: the glare changes from brightest to dimmest every 90°. (1)Examiner tips- Say that the reflected light is polarised. It is the reflection, not the sunglasses, that first polarises the glare.
- State the direction of polarisation (horizontal) and the direction of the transmission axis (vertical). Vague answers such as "the glasses block the polarised light" lose marks.
- Reflected light is completely polarised only at the Brewster angle. At other angles it is partially polarised, so some glare remains.