Hello, I am Pandurang Bhau Gaikar. I teach at Nutan Vidyamandir in Mumbai (Mankhurd). The objective of today’s lesson is to study how we can actually see things
through the action of light ray and the human eye. We see light here, it’s all bright. But how is the route of a light ray? Can anybody tell?
– Straight It goes straight. Okay. Now I have switched on the torch. Can you see it going straight?
– I can see the dot there. Right, we can see the dot there. But then, I can also say that it might be going in
a zigzag manner. For example, I have a laser here and I have pointed it here. What can you see? – Only a dot We can only see a dot, that’s right. But how does the dot travel from here to there?
– Straight It is going in a straight line. But how can you explain it? – If there is a hole in any metal sheet at home and – if a ray passes in through it then we can
it pass in a straight line in the smoke. We can see it at our homes also. If there’s
a small hole in a metal sheet or roof, a light ray comes in from it and you are able to see it. So tell me why can you see it,
there’s a reason for it as well. – Because of dust particles in the air. Right, we can see it because
of the dust particles in the air. Now we’ll study how the light ray
actually passes and what is its route. In the first part, students are able to clearly see how three rays of light
pass in a straight line because of the smoke created. This helps them understand that light rays
pass in a straight line. There are some errors in the book
as only ray diagrams are shown, and hence students are unable
to understand the concept clearly. So to explain that I’ve taken this box. Since I have switched on the laser
and created smoke inside it, students can clearly see how
light rays pass in a straight line, how rays converge when passed through a convex lens or how they diverge when passed through a concave lens. I have a circle of glass here, have a look at it. Give it to her too. Look closely and handle it well,
check which type of circle it is. Now how did the circle feel to your hands? – It was thicker in the middle.
It was thicker in the middle. – And thinner on the sides
And thinner on the sides, right? This kind of glass circle is called lens. And what type of lens is it? It’s a convex lens.
This is called convex lens. In part two of the lesson, we provided
students with different types of lens, and they were able to identify if it is
a convex lens or a concave lens. And when we put these lenses in the box, they
could see with which lens all light rays come together and which lens makes them spread out. Observe it closely. Lens has been put inside. When I put a convex lens inside the box, what happened?
– Rays became one. Yes, once again?
– All the light rays came together at one dot. That’s right! All lights rays come together in
a dot. Which property have you learnt today? – All light rays converge at one point. Property of this lens is that
all parallel light rays converge at a point. In the third part of the lesson, we placed three different
types of human eye sequentially inside the box. Students were able to observe how
the light rays converge on the retina, how sometimes they are formed beyond the retina or
how sometimes they are formed before the retina itself. Through their observations about how different lenses could make the rays fall on the retina, students answered questions by themselves, We have a wooden circle here. What does it look like?
– Human eye It looks like a human eye, right? How we see things is a very interesting topic and now we are going to see the same. Now tell me what is happening here? – All rays are converging at a dot
at the back of the human eye. They are converging at the back dot. Correct! Now, if you think thoroughly, what
made all the rays converge? – Convex lens. Right. That means which type of lens
do we have in our eyes? – Convex
Right, convex! And what has happened to the rays
because of this convex lens? – They’ve converged. Have you seen it closely?
Where did the rays converge? – On the retina. They’ve converged on the retina. Lights rays travel from things to our eyes. From the eyes they pass through the eye lens
to come together at a point on the retina. This concept is passed to the brain via nerve
fibres and brain comprehends the same. Then we understand the thing
when brain analyses the concept. The shape of our eye is generally of this type. We have another wooden eye. Observe it closely. Now I am placing this eye in the box. Have all of you observed it? What is happening now considering the previous eye? – Light rays are converging before the retina. Light rays are converging before the retina.
But I can also see them on the retina also. We can see 3 dots on the retina also. So what is
going to happen because of them being formed before? – The person will not be able to see far off things, he will
only be able to see things which are close. Now tell me one thing, if I tell you to remove your
spectacles and read something written on the board, will you be able to see it? – No So what happens then?
– Everything looks blur. Everything looks blur! If I want to see things clearly,
then what will I need to do? – The convergence point should be on the retina. Right, it should be on the retina. What will happen then? – Clear vision Now how does it look to him?
– Unclear Unclear means?
– …Blur Now tell me which type of lens should I use to
enable this person to see things clearly? – Concave! Why are we using a concave lens? Why not convex? – Convex lens converges all the rays before the retina, – whereas concave lens diverges all the rays on
the rays and they come together on the retina. Correct! So what will happen if I use convex lens? – They will come together much before the retina. But we want them to converge a
little further, on the retina. So we will have to diverge the rays. Now if we diverge the received light rays then
they will come together on retina. Now let’s see how that happens. The main objective of science is observation and conclusion. Hence I asked questions related to observations and
conclusions to students and tried to find out answers from them. This visual impairment is called
near-sightedness. What is it called? – Near-sightedness. Wherein a person can clearly see nearby things, but not the far-off ones. This visual impairment is called near-sightedness. And what is the shape of this eye? Is it same like the previous one? Can anybody tell me? – It is elongated.
It is elongated. Correct! This one is elongated, the previous one was circular. This one is slightly elongated! What happens because of that? Rays converge
before the retina instead of converging on it. Hence which lens have we used?