Senior Physics - Solutions

## Chapter 20. - Optical Instruments - Worked Solutions

### ...to selected Complex Reasoning questions from the OUP text "New Century" Senior Physics - Knowledge, Processes and Reasoning by Walding, Rapkins and Rossiter

Q 31.
The image will need to be at the near point , 25 cm from the eye.

Assume the eye and the lens are 5 cm apart then:
v = 25 -5 = 20 cm and virtual
v = -20 cm
1/v + 1/u = 1/f
1/-20 + 1/u = 1/10
1/u = 1/10 + 1/20
1/u = 3/20
u = 6.67 cm
the coin (stamp) is 6.67 cm from the lens.
Q 32.
You will need a lens that has the object at 25 cm and produces an image at 50 cm. This is so that to the eye the object will appear to be at 50 cm and will be in focus. This person is far sighted and therefore needs a convex lens.
v is 50 and virtual
u = 25 cm

1/v + 1/u = 1/f
1/-50 + 1/25 = 1/f
-1/50 + 2/50 = 1/f
1/50 = 1/f
f = 50 cm
Q 33.
The lamp and the concave reflecting mirrors illuminate the book. Light from the book is reflected from the plane mirror through the lens which focuses the image onto a screen. The plane mirror also reverses the image.
Q 34.
The pin casts a simple shadow on the retina which inverts it. The lens system of the eye is unable to focus on the pin as it is too close.