1. Module 5 Optional Component
Astrophysics

2. Structure of unit 5

3. Light rays from a great distance are effectively parallel to each other.
That is why shadows of trees in sunlight appear parallel.
As a convention we refer to light rays from a distance originating at infinity.

4. Light from the Sun
Light rays from a very distant light source (e.g. the Sun) arrive at the Earth travelling (practically) in parallel.

5. The converging lens.
A simple converging lens is a lens which brings parallel light rays to a focus.
Converging lenses are essentially thicker in the centre than they are towards the rim
The biconvex lens is the simplest because of its symmetry
Plano-Convex
Biconvex
Concavo-convex

6. Light is brought to a focus by a converging lens because the lens is essentially behaving like a collection of prisms.
In reality the theory of lenses applies to “thin lenses” which have a large radius of curvature

7. A converging lens is one which brings parallel light to a principal focus, that is a place where the rays which were originally parallel, cross on the principal axis.
The principal focus of a convex lens is called real. 
.  This means that it can be projected onto a screen. 

8. Finding the focal length of a converging lens.
A lens is placed normal (at right angles to) parallel rays emerging from a ray box or from a distant source, eg the Sun.
The rays converge at the principle focus and the distance between the centre of the lens and the principle focus is the focal length of the lens (f)

9. Producing a real image with a converging lens.
Light from a very distant object e.g. This house arriving practically parallel. Produce an image of the house on the screen as shown.
What do you notice about the image?
Measure the distance between the centre of the lens and the screen.

10. Letter Conventions u v f
Image I
u is the distance from the object to the centre of the lens
v is the image distance from the centre of the lens
f is the focal length of the lens

11. Ray Diagrams For Converging Lenses
We can determine where an image lies in relation to the objects by using a ray diagram. 
We can do this by using two simple rules:
1. Draw a ray from the top of the image parallel to the principal axis. 
This ray bends at the lens axis and goes through the principal focus.

12. Step 3 Draw a ray from the top of the object through the middle of the lens. 
This ray is undeviated

13. Step 4 Where the rays meet, that is where the image is

14. What is the image like if the object is further away than 2F?

15. What is the image like if the object is at 2F

16. What is the image like if the object is between 2F and F? 

17. What is the image like if the object is at F?
The rays emerge parallel and no image is formed.
We sometimes say that the image is at infinity. If the rays manage to cross somewhere in the far distance the image would be real and inverted.

18. What is the image like if the object is less than F?
Here the light rays diverge. They cannot be brought to a focus and cannot be projected onto a screen. But if we look through the lens our eyes see the rays appearing to come from a place behind the object.
This is called a virtual image.
It is magnified and upright. The lens behaves as a magnifying glass.

19. The Lens Formula u v f Real Is Positive Convention
The focal lengths of converging lenses are positive. Those of diverging lenses are negative
distances from lenses to real objects and real images are positive. Distances to virtual objects and virtual images are negative
Image I

20. Question
Calculate the distance of the image produced by an object placed 80cm from a lens of focal length 30cm.
Draw a scale diagram on graph paper
List three properties of the image.