# In any glass, why doesn't its shadow show when near a screen?

$$\hspace{25px}$$ $$\hspace{25px}$$\begin{alignat}{7} & \begin{array}{l} \textbf{Figure 1.}~~ \text{Here shadow is showing, but} \\ \text{not when it is near. I had seen an} \\ \text{explanation, but I am not sure about it.} \end{array} & & \begin{array}{l} \textbf{Figure 2.}~~ \text{Here shadow is showing, but} \\ \text{not when the glass is near.} \\ \phantom{} \end{array} \\ \hspace{20px} & \hspace{300px} & \hspace{25px} & \hspace{300px} \end{alignat}

• Welcome to SE.Physics! I formatted your pictures to be side-by-side using a bit of trickery. It'd be helpful if you could reword the question slightly, perhaps adding a bit more of a question statement to the post itself rather than relying on the question's title to convey it. – Nat Nov 4 '18 at 9:28

## 2 Answers

This happens because glasses are basically lenses and lenses change direction of incoming light, causing it to converge or diverge.

Close to a screen, the effect is insignificant, since the intensity of the converging or diverging light changes gradually, with distance.

As the distance from the screen increases, the effect will become more obvious and will depend on the type of lenses, converging or diverging, and the focal length. At some distances, the light could intensify, at other distances, it may get dimmer and you'll see a shadow.

This is illustrated below for short sighted glasses (concave lenses) and for a magnifying glass (convex lens, similar to reading glasses), for different distances.

With flat glass, you would not observe any of these changes.

I know that a figure would help, but I have no time to draw it. Your eyeglasses are negative (diverging) lenses, used by myopic people. To understand the shadows you must keep in mind that the whole surface is reached by sunlight.

If your spectacle frame did not bear lenses you would see direct sunlight, with only a thin shadow produced by frame. The diverging lens receives a parallel beam of rays from sun and transforms it in a diverging beam. So light energy reaching the lens is distributed over a wider area and this affects the intensity perceived, reducing it.

At the same time, outside the frame's shadow, there is direct sunlight with added part of light diverted by the lens. Both contributions sum up to show a brighter lighting.

When you increase the distance between lens and surface below the area interested by the diverging beam increases too and this explains what is shown in your photos.