Happy New Year 2010

Each moment in a day has its own value.
Morning brings HOPE,
Afternoon brings FAITH,
Evening brings LOVE,
Night brings REST,
Hope you will have all of them everyday.
HAPPY NEW YEAR 2010.

Solar Cooker

Application of curved mirror.

Watch the video.

Find out more :
1. Why do they use dull black pot instead of white shiny pot ?
2 Where is the pot located ?
3. Are you able to draw ray diagram to show principle focus of the mirror?

Curved Mirrors

A curved mirror is a section of a sphere.
If the outside surface of the sphere is reflecting, the mirror is convex.
If the inside surface is reflecting, the mirror is concave.

The characteristics of the image formed can be
i. real or virtual
ii. inverted or upright
iii. larger , smaller or the same size of the object
By drawing ray diagrams, you can find out how the image would be.


Ray diagram --concave mirror1. Draw ray from top of object to the curved mirror. After reflecting off mirror, the ray passes through the focal point. (Purple ray)
2. Draw a ray from the top of the object through the focal point. The ray will be parallel to the optical axis after reflecting. (Green ray)
3. Draw the image as a vertical arrow with its tail on the principle axis and the head at the point where the two reflected ray meet.

Tips: Images formed "behind" the concave mirror are always upright and virtual.

Ray diagram --convex mirror


1. Draw ray parallel to optical axis. After reflection, this ray travels as if it is coming from the far focus. (Purple ray)
2. Draw line from top of object to centre ,C (Red ray).
3. The virtual image is drawn using a dotted line with an arrow.

Tips: Images formed by a convex mirror are always smaller that object.
Hand on :
Take the standard steel spoon in your kitchen, observe images formed at both side of the spoon at difference distance. What can you say about the image formed?

Reflection of light

Reflection occurs when a light ray bounces off a surface.

Terminology :

Incident ray is the incoming light ray.

Reflected ray is the light ray moving away from the surface .

The most important characteristic of these rays is their angles in relation to the reflecting surface. These angles are measured with respect to the normal of the surface.

The normal is an imaginary line perpendicular to the surface.

The angle of incidence, θi is measured between the incident ray and the normal.

The angle of reflection, θr is measured between the reflected ray and the normal.

When a ray of light is reflected, the reflected ray lies in the same plane as the incident ray and the normal.

Law of Reflection
The Law of Reflection states that the angles of incidence and reflection are always equal and
that the reflected ray always lies in the plane of incidence.

When you smile into a mirror, you see your own face smiling back at you. Did you notice that;
i. the size of the image is the same as you (object)
ii. the distance of the image seem as the same distance of you from the mirror.
iii. when you move your left hand, the image will move it right hand
iv. then you image never upside down .



worked example:
Question: An incident ray strikes a smooth reflective surface at an angle of 33◦ to the surface normal. Calculate the angle of reflection.

Answer
Step 1 : Determine what is given and what is required We are given the angle between the incident ray and the surface normal. This is the angle of incidence. We are required to calculate the angle of reflection.

Step 2 : Determine how to approach the problem We can use the Law of Reflection, which states that the angle of incidence is equal to the angle of reflection.

Step 3 : Calculate the angle of reflection We are given the angle of incidence to be 33◦. Therefore, the angle of reflection is also 33◦.

quiz :

Laws of reflection » Quiz Make

Is light rays real ??

In physics we use the idea of a light ray to indicate the direction that light travels. Light rays are lines with arrows and are used to show the path that light travels.


The most important thing to remember is that we can only see an object when light from the object enters our eyes. The object must be a source of light (for example sun) or else it must reflect light from a source (for example the moon), and the reflected light enters our eyes.


Light rays are straight lines with arrows to show the path of light.
Important: Light rays are not real. They are merely used to show the path that light travels.

Did you still remember, light travel in straight line?

Formation of shadows is the evidence that light travel in straight line.



Test Quiz

1. Are light ray real ? Explain.
2. Give evidence to support the statement:" Light travels in straight lines". Draw a ray diagram to prove this.

Merry X'mas

Ripple tank.

Ripple tank

A ripple tank is a shallow glass tank of water used in schools to demonstrate the basic properties of waves. The tank is illuminated from above, so that the light shines through the water.
The ripples on the water show up as shadow on the screen underneath the tank.
All the basic properties of waves can be demonstrated.


A stroboscope, also known as a strobe, is an instrument used to make a moving object appear to be slow-moving, or stationary.




Source http://commons.wikimedia.org/wiki/File:Simple_ripple_tank.svg



The water acts as a lens to produce a pattern of bright and dark regions on the screen (or a piece of white paper).