Every card in Waves, light and sound, Year 8: ray diagrams and the eye
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- On a ray diagram, what must every ray have drawn on it to show which way the light travels?
An arrow
HintWithout it you cannot tell the lamp end from the eye end.
WhyLight travels from a source, or from an object that reflects it, to the eye. The arrowhead shows that direction.
- In a ray diagram, the line drawn at 90° to a mirror at the point where the ray hits
The normal
HintAngles of incidence and reflection are both measured from it.
WhyIt is a construction line, usually drawn dashed. Measuring both angles from it shows that the angle of reflection equals the angle of incidence.
- Where does the image in a plane mirror appear to be?
Behind the mirror, as far back as the object is in front
HintTrace the reflected rays backwards with dashed lines.
WhyThe reflected rays enter the eye as if they had come from a point on the far side of the glass. That is where the brain places the image.
- On a ray diagram for a mirror, why are some lines drawn dashed behind the mirror?
No light really travels there
HintThey are a drawing aid for finding the image.
WhyThe dashed lines extend the reflected rays backwards to find the image. Real rays are solid lines; these imagined ones are dashed.
- Which tool must be used to draw every light ray in a ray diagram?
A ruler
HintLight travels in straight lines.
WhyA wobbly freehand ray gives the wrong angles and puts the image in the wrong place. Accuracy is the whole point of the diagram.
- The light-sensitive layer at the back of the eye, where the image forms
The retina
HintIt lines the inside of the eyeball and sends signals along the optic nerve.
WhyIts cells respond to light and send signals to the brain. The image that forms on it is upside down; the brain interprets it the right way up.
- Which two parts of the eye bend light to focus it?
The cornea and the lens
HintOne is the clear front window; the other sits just behind the iris.
WhyThe curved cornea does most of the bending. The lens then makes the fine adjustment, changing shape for near and distant objects.
- The hole in the centre of the iris that lets light into the eye
The pupil
HintIt looks black, and widens in the dark.
WhyThe iris makes it larger in dim light to let more light in, and smaller in bright light to protect the back of the eye.
- In an eye with blurred vision, where does the light come to a focus?
In front of or behind the retina, not on it
HintA sharp image needs the rays to meet exactly on the light-sensitive layer.
WhyIf the rays meet too early or too late, each point of the object lands as a smudge. Glasses bend the light a little so that it focuses in the right place.
- Which part of a camera does the same job as the retina of the eye?
The sensor (or film)
HintIt is where the picture is recorded.
WhyIn both, a lens focuses light onto a light-sensitive surface. The camera's sensor records the picture; the eye's surface sends it to the brain.
1★ KS3-PHYS-WAV-0026
2★ KS3-PHYS-WAV-0027
3★ KS3-PHYS-WAV-0028
4★ KS3-PHYS-WAV-0029
5★ KS3-PHYS-WAV-0030
6★ KS3-PHYS-WAV-0031
7★ KS3-PHYS-WAV-0032
8★ KS3-PHYS-WAV-0033
9★ KS3-PHYS-WAV-0034
10★ KS3-PHYS-WAV-0035
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