Mark scheme for questions about The Eye:
(a) (i) The light rays change direction.
Change direction is sufficient for the mark
(ii) At the retina
Remember that the retina is where the image in the eye is formed.
(iii) To focus on near objects the ciliary muscle contracts, making the suspensory ligaments slack. The eye lens becomes fatter and more powerful.
To focus on distant objects the ciliary muscle relaxes and the suspensory ligaments pull the lens, making it thinner and less powerful.
The question is really asking two things: first about being able to focus on near objects and second about being able to focus on distant objects. This answer is clear to follow and keeps the explanation for near objects separate from that for distant objects. The key point in each part of the answer is the effect of the ciliary muscle and suspensory ligaments on the shape of the eye lens.
(iv) Cornea
It is easy to forget that the cornea also plays an important part in the refraction and focusing of light in the eye.
(b) (i) Short sight. Diverging lenses used to correct short sight have a negative power.
There is no mark for simply stating short sight. The key to this question is remembering the type of lenses that have a negative power and then relating this to the correction of short sight.
(ii) The eyeball may be longer than it would be for normal sight.
The other possible cause of short sight that you need to remember is the inability of the eye lens to focus.
(iii) f = 1/Power = 1/2.5 = 0.4m
This is clearly set out with substitution shown. Remember that no substitution and a slip with the calculator would mean no marks.
No comments:
Post a Comment