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# Lenses for Edexcel A-level Physics

This page covers the following topics:

1. Convex lenses

2. Concave lenses

3. Ray diagrams

4. Magnification

Lenses are transparent pieces of glass or plastic which are used to refract light. A lens can be convex or concave. A convex lens is thicker in the middle and causes parallel light rays that enter it to converge. The light rays will then meet at a point called the principal focus. The distance between the lens and the principal focus is called the focal length. When an object is placed more than twice the focal length from the lens, the image formed is inverted, diminished and real. When an object is placed between one and two focal lengths from the lens, the image formed is inverted, magnified and real. When an object is placed less than a focal length from the lens, the image formed is upright, magnified and virtual (ie. cannot be displayed onto a screen).

Concave lenses are lenses which are thinner in the middle than at the edges. This causes light rays which enter it to diverge, instead of converge as with a convex lens. The point from which all the light rays seem to be diverging from is called the principal focus. The focal length is identically defined as with convex lenses; it is the distance between the principal focus nadd the lens. Concave lenses produce images which are upright, diminished and virtual.

A ray diagram shows the path that a light ray entering a lens follows to form the image. To draw a ray diagram, two steps must be followed. Firstly, a ray must be drawn from the object to the lens parallely to the principal axis, which when goes through the lens, must pass through the principal focus. Secondly, a second ray must be drawn from the object passing through the centre of the lens. A third additional ray can also be drawn into the ray diagram. The normal at the point which the ray enters the lens is the line perpendicular to the surface at that point. The angle of incidence is the angle between the incident ray and the normal, whereas the angle of refraction is the angle between the refracted ray and the normal.

Magnification is a ratio comparing the size of an object and the size of its image produced by a lens. Magnification can be measured using the following formula: Magnification = image height/object height. Since it is a ratio, magnification is not given with units, however when calculating magnification, the two heights should be given in the same units. A magnified image will give a value for magnification greater than 1, whereas a diminished image will give a value less than 1.

# 1

An object that has a height of 5 cm produces an image of height 100 cm. Calculate the magnification.

Magnification = 100 cm/5 cm = 20.

# 2

A concave lens is often referred to as a negative lens because of its negative focal length nature. Explain what this means.

A concave lens is said to have a negative focal length nature due to the fact that the image appears on the same side of the lens as the object, rather that on the other side of it.

# 3

Draw a diagram for a light ray entering a concave lens, labelling the normal, the angle of incidence and the angle of refraction.

image

# 4

An inverted, magnified and real image of an object is formed by a convex lens. State the distance between the lens and the object.

For the image to be inverted, magnified and real, the object must be between one and two focal lengths away from the lens.

# 5

Draw a diagram for a concave lens whose focal length is 1.5 cm for an object placed 4 cm from the lens with height of 2 cm.

image

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