Edexcel IGCSE Physics D Light and Sound Questions

For 1 mark:
- light refracts / bends / changes direction / changes speed / changes wavelength

For 2 marks:
- light refracts / bends towards normal
- light slows down
- wavelength decreases

normal;

$\sin (\mathrm{c})=1 / \mathrm{n} ;$

Notes: allow in words and
rearrangements

- substitution
- rearrangement
- evaluation

Example:
- sin(c) = 1 / 1.6
- c = sin^-1(0.625)
- c = 39 degrees

Accept:
- 38.68...

- total internal reflection at the glass/air boundary
- angle of incidence = angle of reflection

Accept:
- ECF from 10(a)(iv) that would suggest refraction, e.g. if answer to 10(a)(iv) is greater than 50 degrees

Marking guidance:
- Judge by eye.

Correctly reflected ray of light drawn at A.

Marking guidance:
- Judge angle of reflection = angle of incidence by eye.
- Allow dotted lines or lines without arrowheads.
- Ignore lines inside the block.

- angle of incidence = 60 degrees
- angle of refraction = 31 degrees

Accept:
- incidence 59-61 inclusive
- refraction 30-32 inclusive

n = sin(i) / sin(r)

Accept:
- words and rearrangements

- substitution
- evaluation

Example:
- n = sin(60) / sin(31)
- n = 1.68

Accept:
- ECF from 9(a)(ii)

Any three from:
- take repeat readings at a specific angle
- vary angle of incidence
- find mean values for one angle i / mean refractive index
- plot graph of sin(i) against sin(r)
- find refractive index from gradient of graph

Ignore:
- bald repeat and average
- repeat investigation

straight ray bent towards normal at the air-water boundary;
correct angle of refraction by eye;

judge straightness of ray by eye

refractive index = sin i / sin r

Accept:
- rearrangements and standard symbols, e.g. n = sin i / sin r

Marking points:
- substitution
- evaluation to at least 3 significant figures

Example:
$$n=\frac{\sin45^\circ}{\sin33^\circ}$$n=1.30

Do not accept:
- 45/33
- 1.36

Marking guidance:
- reverse argument scores 1 mark only, e.g. calculating i or r using n
- allow $1.298\ldots$
- condone 1.29

the angle of incidence (in the slower medium);
above which gives total internal reflection OR gives an angle of refraction of 90^;
allow ideas if clear from diagram
allow TIR for total internal reflection

sin c = 1 / n

Accept:
- rearrangements and mixture of symbols and words

- substitution
- rearrangement
- evaluation

Example:
- sin c = 1 / 1.3
- c = sin^-1(1 / 1.3)
- c = 50 degrees

Accept:
- 50.3
- 50.28...

single straight ray reflected downwards
from water surface;
angle correct by eye;

any ray above water surface
scores zero