Power of your reading glass

 In earlier videos, we explored ray paths with regular lenses, acrylic lenses as well as lenses made from plastic. What if we don't have access to these lenses ? 

We can use reading glasses to explore ray paths for convex lenses. Reading glasses are spherical glasses with power ranging from 0.25 to 4 diopters. Lenses in some prescription glasses have cylindrical factor as well but we will not cover that as of now.

Let us find out the power of reading glasses with the help of two lasers. Two lasers are projected in parallel on the screen. Paper strip marked with distances in centimeters can be used to find out focal length. Details about the setup, I will talk later. 

As per this prescription, power of both lenses is +1.5 diopter. Lens is spherical. We will not cover cylindrical lens here.  As per formula, focal length of these lenses should be 1 upon power that is 1 meter divided by 1.5  or 100 divided by 1.5 which is approximately 66 cm. 

We will place one part of the glass or lens  in the path of parallel rays coming from lasers. Distance between two rays will change as we move the screen towards and away from the lens. At one point ,rays merge. The distance at this point is nothing but the focal length of the lens. Our calculated value of 53 centimeters is less than the calculated value of 66 cm. This can be attributed to homely setup and not so perfect alignment of laser rays.

We can find out the focal length of other lens as well as turn the glasses around and explore if it has any effect on the focal length.

I visited a local shop to find out the power of old glasses lying around.  Power of the lens for the right eye is +2.0. There is some cylindrical correction as well. We will ignore it for now. 

Our calculated focal length is 40 centimeter which is different from the one found by instrument which is 50 cm. Or 2 diopters . 

 

This is a bifocal lens. Upper part of the lens has a power of +1.5 diopters while the lower part has a power of 4 diopters. 

As we move the lens up and down in the parallel path, we can observe movement of dots on the screen. Dots converge for the lower part while they are some distance apart for the upper one.

Just like a regular lens, rays travelling parallel converge at the focal point. These rays diverge again after the focal point as can be seen here.

Effect of two lenses placed next to each other can also be observed. When a lens with +0.75 power is placed next to the lens with + 0.5 power, effective power goes up. This results in reduced focal length of combination as can be seen here.

I measured the power of this regular convex lens. It is +18 as per the instrument. When placed in front of the lasers, rays merged at a distance of 5 cm from the lens. 

Sunrays

Instead of laser light, we can use sunlight as well. Let us use this wooden stick as a slider and a small platform made from corrugated sheet. This paper has two pinholes through which sunlight will enter. These two rays are projected on the sliding screen. Let us test it inside the room. It's 8 oclock in the morning and I can capture a small beam of light coming through the window. I will use a rubber band to hold the reading glasses in place. Power of this reading glass is +1.5. Incoming rays pass through one of the glasses and project on the screen. As we slide the screen away from the glasses, two rays merge. We see only one circle instead of two. This distance is the focal length for the reading glass we are using. In this case it is 55 cm. 

Observations are not exactly matching the calculation but this setup enables students to visualize various aspects of the ray path associated with convex lens using regular reading glasses. 

Do try this setup for bifocal glasses as well !

Thank You.

Peripheral Vision

 Healthy eyes do more than just allow you to focus farther as well nearer. 

Eyes also help you to sense things which are not in your direct sight. This is also known as side vision or peripheral vision. This is important during cycling, reading or playing sports. When you look at something, you use central vision to focus on details while peripheral vision to know more about the surroundings.

Let us perform this activity to understand peripheral vision better.

We will need some color markers and a paper strip. Let us make a small paper roll from the strip. Small Paper rolls are used to write combinations of color, shape and text.  We need to mark angles as well. These can be written on a piece of paper. 

Tejas will help me in setting things up as well as doing this activity. 

Let us place a 90 degree mark right in front of his right eye.  With the help of his fist, Tejas approximates the 80 degree mark, a step of 10 degrees. With his fingers extended all the way, he marks 60 degrees, an increment of 20 degrees. 

With all angles marked, Tejas looks straight at his thumb of left hand placed on the table. He stretches his right hand all the way behind him. 

Let me place a small paper roll marked with Shape, Color and Text on his right hand thumb. He is not aware of what is written on it. 

He will slowly move his right hand towards the left hand placed on the table. While doing this, he will try to identify 4 things with his right eye. Motion, Color, Shape and Text. This will happen at various angles during the movement of his hand. 

He detects motion first, followed by color, shape and finally text. This activity can be repeated many times and an approximate angle for each can be calculated.

Same can be done with another eye as well. 

Now let us understand why he is able to read text only when it is almost in front of his eye. 

This is the internal structure of the eye. 

Let us consider his right eye for now. 

The cornea is the clear outer layer at the front of the eye. It allows light to enter the eye. It provides approximately 65 to 75 percent of the focussing power of the eye.

The remainder of the focusing power of the eye is provided by the crystalline lens, located directly behind the pupil.

The retina is the sensory membrane that lines the inner surface of the back of the eyeball. It has special cells known as photoreceptors.

There are two types of photoreceptors - rods and cones. 

Rod photoreceptors detect motion, provide black-and-white vision and function well in low light. Cones are responsible for central vision and color vision

Rods are located throughout the retina; cones are concentrated in a small central area of the retina called the macula. At the center of the macula is a small depression called the fovea. The fovea contains only cone photoreceptors and is the point in the retina responsible for maximum visual acuity and color vision.

Our eye is like a pinhole camera. 

Rays reach this region where only cone cells are present. All other rays reach regions where there are mostly rod cells. 

This is why Tejas could read a word only when it is in front of his eye. 

Do try this at home and check your peripheral or side vision.

Thank You.