Lesson: Light
Topic 1: Laws of Reflection

You may have observed the image of the sun in the windows of distant buildings during sunset or sunrise. However, the same image is not seen in the windows of distant building at midday. Can you explain why?

As the sun is relatively low in the sky during sunrise or sunset, the rays of the sun strike the window and reach our eyes, which are at a similar height to the window. At midday, the sun is very high, so rays from the sun that strike the window reflect downward and so we are not able to see the reflection.

How does a ray of light follow the first law of reflection if after reflection the individual rays for a number of incident parallel rays scatter in many different directions?

According to the first law of reflection, the angle of incidence is equal to the angle of reflection. When a surface is rough, it can be considered to be a collection of numerous tiny plane mirrors oriented at different angles. Thus, individual reflected rays will scatter in different directions, but they can be considered to be following the first law of reflection individually.

Most magazines have glossy pages, whereas newspapers have a rougher surface. Would it be easier to read from rough pages or glossy pages? Explain your answer.

It is much easier to read from rough pages as they undergo diffused reflection. Glossy pages result in regular reflection and cause a glare on our eyes. The reader mostly sees an image of the light source which illuminates the page, rather than seeing the text and pictures on the page.

Can you see the objects in a dark room? Can you see objects outside a dark room? Explain.

We see objects only when light reflected by that object reaches our eyes. In a dark room, there is no light that can be reflected by any object and so we are unable to see objects. But if there is light outside the room, we can see those objects, since they will reflect light that will reach our eyes.

In which direction will a ray of light falling along the normal to a mirror be reflected?

A ray of light falling normally on a mirror means that it is falling on the mirror at

${90}^{\circ }$ to the mirror surface. Therefore, the incident ray and the reflected ray both make an angle of $0^{\circ }$ with the normal as per the first law of reflection and the ray, upon reflection, retraces its path.

Define reflection of light and state the laws of reflection.

Reflection is a phenomenon in which a ray of light falls on a surface and returns to the same medium in a different direction.

The laws of reflection are as follows:

1. When a ray of light reflects off a surface, the angle of incidence is equal to the angle of reflection.
2. The incident ray, reflected ray and the normal always lie in the same plane.

What is the angle of incidence?

The angle between the normal and the incident ray is called the angle of incidence.

What is the angle of reflection?

The angle between the normal and the reflected ray is called the angle of reflection.

Explain diffuse reflection?

When a set of parallel rays undergoes reflection, but the reflected rays are not parallel to each other, the resulting reflection is known as diffuse reflection.

How are we able to see an object which is not self-illuminating?

An object that is not self-illuminating reflects light that is incident on it. We are able to see these objects since our eyes receive light reflected from the object.

Topic 2: Image Formation

You might have noticed that emergency vehicles, such as ambulances often have text in the front written in reversed lettering. Why?

Vehicles drivers on a road normally view the ambulance in their rear-view mirrors, so whatever they see will be a mirror image. Thus, if the text on the front of an emergency vehicle is written in reversed lettering, the image seen by the driver in the rear-view mirror will appear in proper orientation.

List five characteristics of an image formed by a plane mirror.

The five characteristics of an image formed by a plane mirror are as follows:

1. Image is erect.
2. Image is virtual.
3. Image distance from the mirror is equal to the object distance from the mirror.
4. Image height is same as object height.
5. Image is always formed behind the mirror.

List three uses of a plane mirror.

The three uses of a plane mirror are as follows:

1. Plane mirrors are used as a looking glass.
2. Plane mirrors are used to construct periscope which is used in submarines.
3. Plane mirrors are also used to make kaleidoscope, a toy which produces beautiful patterns from coloured paper, pieces of glass or small coloured beads.

Can the image formed by a plane mirror be projected on a screen?

No, images formed by a plane mirror can never be projected onto a screen, as light rays do not actually reach the image. The image is a virtual image positioned behind the mirror.

Three objects A, B and C are placed in front of a plane mirror as shown. Can Priya see the images A, B, C on the mirror? Justify your answer.

A plane mirror forms a virtual image behind the mirror. The image distance between the mirror and the image distance will always be equal to the distance between the mirror and the object. Priya can see the objects placed at points A and B but not the object placed at point C as per the image below. Object C is out of range of Priya’s visibility.

The image of an object is formed by two plane mirrors. One mirror is twice the size of the other. In which mirror will the image be bigger and why?

The image of an object will be the same size as the object, irrespective of the size of the plane mirror.

Define lateral inversion?

When an image is formed by a plane mirror, the left-to-right orientation of the object is reversed in the image. This is known as lateral inversion.

Topic 3: Functioning of the Human Eye

How you can take care of your eyes?

We should take the following measures to look after our eyes:

i.        Wash our eyes two or three times a day

ii.      Never touch our eyes with dirty hands

iii.    Never rub our eyes

iv.    Read books and watch television from an appropriate distance

v.      Consult an ENT specialist in case of redness or irritation

Describe the construction of a kaleidoscope.

A Kaleidoscope is made by joining three plane mirror strips to form a triangle.  A piece of hollow cardboard tube is wrapped around the mirrors. On one side of the cardboard tube we place small pieces of plastic or glasses of different colours, wrapped between two transparent sheets. The other side is closed with a transparent plastic sheet. When we look through the plastic sheet, we see beautiful coloured patterns formed as a result of multiple reflections of the pieces on the mirrors.

Why do we blink our eyelids?

Blinking allows the eyelid to help keep the eye clean and protects our eyes from becoming dry. Another benefit of blinking is that it can stop foreign bodies from falling into our eyes.

What happens to your pupils as you enter a dark room from sunlight? Why?

As we enter a dark room from sunlight, we are unable to see things for a little while. This is because the pupils will have closed in the sunlight. As the pupils open in the dark room, more light enters the eye and we can see things again.

Once a ray of light enters the eye, which part of the eye focuses the light ray on the retina?

Once light enters the eye, the lens focuses the light on the retina.

How is a rainbow formed in nature? And in lab by a prism?

In nature, raindrops break sunlight into the combination of colours seen by the human eye. This is because every raindrop acts like a prism and breaks the light into rainbow colours. A prism does the same thing, breaking white light into its different constituent colours.

What causes the blind spot?

At the point in the eye where the optic nerve connects to the retina, we don't have rods or cones. This place is called the blind spot. Thus, images falling on this portion of the retina or the blind spot are not visible to us.