Convex and concave lenses, ray diagrams, real and virtual images, and magnification = image height รท object height.
What you will learn
- Describe how convex and concave lenses refract light
- Use a ray diagram to find an image and say whether it is real or virtual
- Calculate magnification, with heights in mm or cm
Two kinds of lens
A lens refracts light to form an image.
- A convex (converging) lens is fatter in the middle. Rays parallel to the axis are brought together at the principal focus F.
- A concave (diverging) lens is thinner in the middle. Parallel rays spread out as if they came from a principal focus on the near side.
- The focal length is the distance from the centre of the lens to F. A shorter focal length means a more powerful lens.
To find the focal length of a convex lens, point it at a distant object, such as a window across a darkened room, so the rays arriving are almost parallel. Move a screen until a sharp image forms on it, then measure from the centre of the lens to the screen. Repeat and take a mean.
In diagrams a convex lens is a line with outward arrowheads, a concave lens a line with inward ones.
The rest of this lesson and its 10 practice questions are locked.
In this lesson
- Two kinds of lens
- Ray diagrams
- Magnification
- Try it: Move the object, watch the image
- Try it: Read a ray diagram
10 practice questions, marked as you go; about 25 minutes in all.
Key terms
Convex, Principal focus, Concave, Focal length, Virtual, Real
In this unit: Lenses, visible light, sound and black bodies
Reflection and the glass block practical, total internal reflection and optical fibres, colour and filters, lenses and magnification, sound and echoes, ultrasound and seismic waves, and black body radiation (Physics only).