Uses of Lenses
Magnifying Glasses
Magnifying Glasses
If the object is placed closer to the lens than the focal length, the emerging rays diverge and a real image is no longer formed
When viewed from the right-hand side of the lens, the emerging rays appear to come from a point on the left
This point can be found by extending the rays backwards (creating virtual rays)
A virtual image will be seen at the point where these virtual rays cross
A Lens Close to an Object Forms a Magnified Image
A virtual image is formed by the divergence of rays from a point
In this case the image is:
Virtual
Enlarged
Upright
Using a lens in this way allows it to be used as a magnifying glass
When using a magnifying glass, the lens should always be held close to the object
Correcting Sight
Correcting Sight
Converging and diverging lenses are commonly used in glasses to correct defects of sight
Converging lenses can be used to correct long-sighted vision
Diverging lenses can be used to correct short-sighted vision
Correcting Short-Sightedness
People who are short-sighted have eyes that are 'too large'
This means they cannot see things that are far away, and only see things that are close to them
This is because the eye refracts the light and brings it to a focus before it reaches the retina
In other words, the focus point is in front of the retina at the back of the eye
This can be corrected by using a concave or a diverging lens in front of the eye
This causes the light to focus further back, at the retina
Correcting Long-Sightedness
People who are long-sighted have eyes that are 'too small'
This means they cannot clearly see things that are close, and can only clearly see things that are far away
This is because the eye refracts the light rays and they are brought to a focus beyond the retina
In other words, the focus point is behind the retina at the back of the eye
This can be corrected by using a convex or converging lens in front of the eye
This causes the light to focus further forward, at the retina