Scale diagrams for convex lenses | Exit Quiz | Oak National Academy

Which of these statements are correct about rays of light that hit a lens?

• Rays from a nearby object are diverging.

• Rays from a nearby object are converging.

• Rays from a nearby object are very nearly parallel.

• Rays from a very distant object are converging.

• Rays from a very distant object are very nearly parallel.

Answer explanation

Rays diverge as they leave the surface of an object. Rays from an object far away have become almost parallel by the time they hit the lens.

This scale ray diagram shows an object (an arrow), a convex lens and an image. Which of these words describe the image?

• diminished

• inverted

• life-sized

• magnified

• upright

Answer explanation

Compared with the object, the image is upside-down (inverted) and larger (magnified).

When a lens forms an image on a screen, which of these statements correctly describe the image distance?

• It always equals the object distance.

• It is the distance between the image and the lens.

• It is the distance between the image and the object.

• It is the distance needed between the screen and lens for the image to be sharp.

Answer explanation

The image distance is the distance between image and lens, while the object distance is the distance between object and lens. These are equal only if the object is twice the focal length from the lens.

The diagram shows rays of light from an object entering an eye through its lens. The rays are focused onto the back of the eye which acts as a screen. How does the eye focus on a more distant object?

• No change is needed because the back of the eye is at the principal focus.

• The lens becomes fatter so that it has higher power.

• The lens becomes thinner so that it has lower power.

• The eye becomes longer so that the screen is further from the lens.

• The eye becomes shorter so that the screen is closer to the lens.

Answer explanation

Light rays from a more distant object are less diverging, so they do not need to be refracted as much to bring them together. A lower power lens is needed to refract less, so the eye muscles stretch it to make the lens thinner and less curved.

In this ray diagram, the principal focus on each side of the convex lens is shown as a cross. Three rays from the top of the object (an arrow) are shown. Match each ray with its description.

• Ray P

• passes through centre of lens without being refracted

• Ray Q

• passes through principal focus, then bends parallel to principal axis

• Ray R

• parallel to principal axis, then bends to pass through principal focus

Answer explanation

These are the three principal rays you can draw when making a scale ray diagram for a lens. If you draw rays coming from the top of the object, they meet at the top of the image. You only need to draw two of the rays to locate the top of the image.

A student tries to use a convex lens to make an image of a bright object. He varies the distance between the screen and lens but finds no image. Which of the following are possible explanations?

• The object is too close to the lens.

• The object is too far from the lens.

• The rays are converging when they leave the lens.

• The lens cannot refract the rays enough to make them converge.

• The student has placed the screen between the lens and the object.

Answer explanation

The screen needs to be on the other side of the lens from the object. Also, the object must be further than the focal length from the lens. If it is closer, its rays are diverging too much for the lens to make them converge.