Reflection

REFLECTION

The Arabian physicist, Alhazen (c.965–1038), applied mathematics to his study of plane and curved glasses and mirrors. He showed that spherical mirrors could not bring parallel rays to a sharp focus – spherical aberration – and that light travels more slowly through a more dense medium, causing the bending at interfaces between media of different density.

1. Laws of reflection

there are 2 laws of reflection

1. the angle of incidence from the normal is equal in size to the angle of reflection.
2. the incident ray, reflected ray and normal are all in the same plane

The Normal is an imaginary line that crosses the line of the mirror at right angles to the mirror surface, at the point the light (ray) strikes the mirror.

An applet of the laws of reflection

http://www.freezeray.com/flashFiles/Reflection1.htm

Demonstration using ray box or laser or other suitable method.

how could you demonstrate and prove for yourself that these laws are correct?

2. Mirrors

images formed by plane mirror

Plane Mirror Reflection

In a plane mirror the image is as far behind the mirror as the object is in front of the mirror

and spherical mirrors.  The Laws of reflection are not changed or forgotten, they just operate the same on a curved surface So we have to keep this in mind when we are drawing ray diagrams of light hitting these curved mirrors .

a diagram here describing the points would be useful

I cut some pieces of transparent plastic ruler and put them in the front of the ray box where the slits would go. Students can see an image of numbers etc.I think that for the actual experiment it is easier to see when the image is sharp with gauze

Simple exercises on mirrors by ray tracing or use of formula.

Ray Diagrams

Rules for drawing these ray diagrams

1. If the ray comes in along the normal, it leaves along the normal
2. If a ray comes in parallel to the normal it leaves passing through the focal point
3. If the ray travels through the focal point, it leaves back parallel to the normal
4. Any ray that passes through the center of curvature passes back along that line.
5. If a ray strike the mirror at the pole, then the angle of reflection = the angle of incidence

Practice

use this image to help Concave

draw the ray diagrams of objects placed at .....

So one of the keys to concave mirrors is that the image of an object very far away (at infinity) is at the focal point, the opposite of that is an object located at the focal point of the concave mirror projects its image at infinity ... if you don't understand that look at the following link

Concave mirror as used in headlights and other long beam projectors. The bulb is put at the focal point of the mirror so the light that strikes the mirror leaves the mirror parallel to the principal axis.

camera looking at a concave mirror (vanity mirror), solar cookers also use concave mirrors where the item to be cooked is at the focal point again

u is the distance from the mirror to the object

v is the distance from the mirror to the image

Knowledge that f is the focal length, this is a constant for each mirror!!!

m stands for magnification! Real-is-positive sign convention.

Therefore a virtual image gives a negative value for v!

Do questions 1-10 at the end of this section of the chapter,

Corrections to be done in school

Optics puzzle .... definitly worth checking out

http://vnatsci.ltu.edu/s_schneider/physlets/main/optics_p01.shtml

so the outcomes of the ray diagrams are drawn accurately using the rules, where the rays meet (the outcomes of all the rays should meet at the same point) is where the image is.

Inside f the image is Virtual !

Ray diagrams for convex mirrors

In the case of convex mirrors again the Laws of Reflection still apply.

Rules for drawing Convex ray diagrams

1. If a ray strikes the mirror at the pole, then the angle of reflection = the angle of incidence
2. If the ray comes in along the normal, it leaves along the normal
3. If a ray comes in parallel to the normal it leaves as if it passed through the focal point on the other side.
4. If the ray travels towards the focal point, it leaves back parallel to the normal
5. Any ray that passes towards the center of curvature passes back along that line.

Convex

You do not need to complete the same ray diagrams as you did for the concave mirrors, but you should do at least 2 diagrams, one close to the mirror and one further away

http://physics.slss.ie/resources/convex%20mirror.swf

The image in a convex mirror is always

• Virtual
• Diminished (the closer the object is to the mirror the larger the image)
• Erect

Do question 1-8 pg 26

Practical uses of spherical mirrors

Title

Measurement of the focal length of a concave mirror.

Apparatus

Concave Mirror, Stand, Retort stand, a crosshairs (make one yourself), a ray box.

Method

1. Find an approx value for the focal length by focusing an image of a distant object on a screen (nothing more than a sheet of paper). Measure the distance from the mirror to the image ... the approx focal length
2. Set up the apparatus as in the diagram, Keep the distance < approx focal length found in 1
3. Make a screen using the Card in the retort stand
4. Adjust the position of the screen until you see a clear image of the cross-hairs on the screen.
5. Measure u (mirror to object), v (mirror to screen), write down the values
6. Change values of u re-do at least 5 times (minimum of 6 points)

Diagram

What did you see or notice

Safety Concerns

What did you see or notice

Results / Observations

Maths

find the average value for f

When using the formulae be accurate use more than the usual levels of digits

On graph paper plot 1/u on one axis and 1/v on the other axis, note the values where they cut the axis and use these as 1/f, find an average of the 2 values and use the recipricol of this as the focal length

Conclusions

What did you see or notice

Sources of Error 