Optics

Question 1

Focal points

Answer 1

• A parallel beam of light from a distant object can be the converged to a focus in the focal plane.
• If the beam is parallel to the principal axis, the light is focused to the focal point on the principal axis.
• A diverging beam from a near object will focus at a point beyond the focal plane.

Question 2

What is a camera?

Answer 2

• A camera is a box with a lens at one end and a film at the other.
  
  • Light from an object passes through the lens and is focused on the film.
  • The size of the image is smaller than the object.

Question 3

What happens in a projector?

Answer 3

• In a projector the film is placed closer to the lens than the object is to the camera.
  
  • Light from a bulb passes through the film and the lens and is focused on the screen.
  • The size of the image is larger than the object.

Question 4

The paths of two rays from the top of the object are as follows:

Answer 4

• The ray parallel to the principal axis refracts through the focal point
• The ray through the centre of the lens is not deviated.

Question 5

Cameras and convex lenses

Answer 5

• A camera forms a real, inverted, small image on the film.
• Simple lens cameras have a fixed lens. This only produces a sharp image for one object distance. Better quality cameras have an adjustable lens distance. This means the image is sharp whatever the object distance.

Question 6

What does a shutter do?

Answer 6

The shutter opens and closes to allow light on to the film.

Question 7

What is the aperture?

Answer 7

The aperture is an adjustable hole allowing different amounts of light onto film.

Question 8

What does a projector form?

Answer 8

• Projectors form a large, inverted real image on a screen.
  
  • The lens and/or screen can be moved to produce a sharp image.
  • The condensor lenses make sure the film is uniformly illuminated.
  • The curved mirror reflects light to the condensor lenses.

Question 9

Magnification Eq.

Answer 9

magnification = image size/object size

Question 10

Virtual images

Answer 10

The image formed by a magnifying glass is the right way up and a virtual image - it cannot be projected onto a screen because no light passes through it.