1. ## I just had one of those stoner questions pop into my head

It might just be the lateness of the hour, but I can't seem to wrap my head around this:

Why do things look smaller when they're farther away? The light coming from them expands outward in a sphere, and we catch a few photons of it, but wouldn't something at the edge of that sphere appear the same size, or larger?

2. I thought about this once, and I realised that the further things were away from you the larger the sphere surface for objects that could be placed in this sphere, so the more of any same object could be placed in this surface area, so they would have to look proportionately smaller.

3. Member
Join Date
Jun 2002
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32
We don't really see how big something is, we see the angle at which it appears to us, transported by the photons coming from the object to our eyes. Most photons from the object will just miss our eyes. With lenses, for example, one can get photons that would usually miss our eyes be directed towards them, so in that case the photons from the object reach our eyes from greater angles, making the object appear larger.

4. Let's keep the basic explanation simple. Light from each point on the object goes in straight lines to the eye's pupil. When the object is moved farther away, the rays from the extremities subtend a smaller angle, and thus make a correspondingly smaller angle in going from the pupil to the retina. This makes a smaller image on the retina. In an exercise like this a sketch is worth many words. If you are still having trouble visualizing it, I can try attaching a sketch.

5. No, that makes sense. Thanks :-)

6. Often, taking the opposite approach helps in answering a question. If distant objects appear larger, then close objects would appear smaller. How small would our thumb look as it comes closer to the eye? How could our thumb block our vision if it becomes super tiny?

7. Think of it as a right-angle triangle. If you keep the upright side the same length but extend the base, you'll see how the ratio changes. Now, if you think of the base as the radius of a circle, you'll realize it's also smaller compared to the circumference of that circle.

Optics is about light cones converging at the focus. It's not 1 right-angle but two, though more precise calculations are done with trigonometry.

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