# Light refraction and lensesPage 3

#### WATCH ALL SLIDES

Concave lenses produce only virtual images.

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## Finding the focal length

Rays coming into a lens from a distant object are almost parallel and form an image very close to the focus.

We can then measure the distance from lens to image to determine the focal length of the lens.

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## Images in a Convex and Concave Lens Prac

AIM: To investigate the image formed by different convex lens and concave lens

Complete Convex prac – using 2 different convex lenses

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Questions

CONVEX –

1) What happened as the object was brought closer to the lens?

2) When does;

a a real image (on a screen) is obtained

b a virtual image (one that cannot be ‘caught’ on a

screen) is obtained

c no image is obtained

CONCAVE -

1) Assess whether it is possible to form a real image (one that may be ‘caught’ on a screen) using a concave lens.

2) Explain how the image changes as the object-to-lens distance is varied.

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## Optical instruments

Telescopes

Telescopes make small, far objects appear larger.

Two lenses are used.

The objective lens produces a real, inverted image just inside the focus of a second lens, called the eyepiece lens. The image produced by the first lens now acts as the object for the second lens. Because the first image is inside the focus of the second lens, the second image (the one seen by the telescope user) is virtual and enlarged compared to the first one

The thinner the first lens (objective lens), the larger the first image. But thin lenses have longer focal lengths—this is why telescopes are long instruments.

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Eye ball prac

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