# Lenses aqa unit 5 option a

Notes on main points of 'Lenses notes'

HideShow resource information

## Glossary

Principle focus = The point onto which the rays parallel to the principle axis converge.

Focal length = the distance between the lens axis and principal focus

Converging lens = lens that brings light rays together, changes direction of light rays by refraction

Real image = light rays actually there, image can be captured on screen.

Virtual image = light rays aren't where images appear to be, images can't be captured on screen.

Focal point = the point where light rays originating from a point on the object converge.

1 of 7

## Lenses

Each lens has:

• principal focus
• focal length

Converging lens:

• a more powerful (thicker) lens converges the rays more strongly and will have a shorter focal length.

Power of lens = 1 divided by the frequency.

• power of lens = diopmeters, D
• frequency = meters.

P= (1/f)

2 of 7

## Formation of image on a converging lens

Converging lens can form real and virtual images.

• Real image = original image and magnified image on different sides of lens
• Virtual image = original and virtual images on same side of lens.

(you need to know ray diagrams for aqa syllabus so get them from somewhere)

3 of 7

## ray diagrams

you need to be able to draw a ray diagram that includes:

• parallel light rays
• lens
• principle axis
• lens axis
• focal length
• principle focus
• focal plane
4 of 7

## lens equation

(1/u) + (1/v) = (1/f)

• 1/u = distance between object and lens axis
• 1/v = distance between image and lens axis
• 1/f = focal length
5 of 7

## astronomical telescope

need to be able to draw a ray diagram consisting on two converging lens

has to include:

• rays from object to infinity
• objective lens
• eye piece lens
• focal point
• focal length
• principle axis

a telescope, in normal adjustment, is set up so that the principle focus of the objective lens is in the same point as the principle focus of the eye lens, so the final magnified image appears at infinity.

6 of 7

## angular magnification in normal adjustment

m= (angle subtended by image at eye) /(angle subtended by image and unaided eye) = Ѳi/Ѳo

• eye lens acts as a magnifying glass on to image to form a magnified virtual image
• real image formed on focal plane
• image appears closer than object because telescopes magnify angular diameter of objects.
7 of 7