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.
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