Convex Lens
A convex lens is thicker at the centre than at the edges.
Convex lenses are thicker at the middle. Rays of light that pass through the lens are brought closer together (they converge). A convex lens is a converging lens.
When parallel rays of light pass through a convex lens the refracted rays converge at one point called the principal focus.
The distance between the principal focus and the centre of the lens is called the focal length.
Use of Convex Lenses – The Camera
A camera consists of three main parts.
The rays of light from the person are converged by the convex lens forming an image on the film or charged couple device in the case of a digital camera.
The angle at which the light enters the lens depends on the distance of the object from the lens. If the object is close to the lens the light rays enter at a sharper angled. This results in the rays converging away from the lens. As the lens can only bend the light to a certain agree the image needs to be focussed in order to form on the film. This is achieved by moving the lens away from the film.
Similarly, if the object is away from the lens the rays enter at a wider angle. This results in the rays being refracted at a sharper angle and the image forming closer to the lens. In this case the lens needs to be positioned closer to the film to get a focused image.
Thus the real image of a closer object forms further away from the lens than the real image of a distant object and the action of focusing is the moving of the lens to get the real image to fall on the film.
The image formed is said to be real because the rays of lighted from the object pass through the film and inverted (upside down).
The Magnifying Glass
A magnifying glass is a convex lens which produces a magnified (larger) image of an object.
A magnifying glass produces an upright, magnified virtual image. The virtual image produced is on the same side of the lens as the object. For a magnified image to be observed the distance between the object and the lens must be shorter than the focal length of the lens.
Magnification
The magnification of a lens can be calculated using the following formula;
As this is a ratio between heights it has no units. A magnification of 2 means the image is twice the size of the object and a magnification of 1 indicates an image size being the same as the object size.
Concave Lens
A concave lens is thinner at the centre than at the edges.
Convex lenses are thinner at the middle. Rays of light that pass through the lens are spread out (they diverge). A convex lens is a diverging lens.
When parallel rays of light pass through a concave lens the refracted rays diverge so that they appear to come from one point called the principal focus.
The image formed is virtual and diminished (smaller)
Use of lenses for correcting vision
The diagram below shows the cross section of a human eye.
Light from an object passes through the cornea which is a transparent dome like structure covering the iris. The light rays are refracted by the cornea onto the lens. The light rays are refracted a second time whilst passing through the lens and focussed onto the retina – the light sensitive part of the eye. The image formed on the retina is inverted (upside down) and real (the light rays travel through the image).
The image is interpreted the right way up by the brain which is connected to the eye via the optic nerve.
The shape of the eye is very important in ensuring the objects we see are in focus. A person with normal vision can focus clearly on objects both near and far, this is because light from the object is precisely focussed onto the retina at the focal point. However, for some people focussing on objects far away or close can result in a blurred image forming. These defects in vision are referred to as long and short sight.
Long sight
A person who is long sighted can focus clearly on distant objects but cannot focus on near objects. This is because the eyeball is too short. Light from near objects is focussed at a point behind the retina resulting in a blurred image.
This defect can be corrected by wearing a convex (converging) spectacle lens. The rays of light from a near object are converged before entering the eye so that the cornea and eye lens can direct the focal point onto the retina.
Short sight
A person who is short sighted can focus clearly on near objects but cannot focus on distant objects. This is because the eyeball is too long. Light from distant objects is focussed at a point in front of the retina resulting in a blurred image.
This defect can be corrected by wearing a concave (diverging) spectacle lens. The rays of light from a near object are diverged before entering the eye so that the cornea and eye lens can direct the focal point onto the retina.
