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### Course: Physics library>Unit 15

Lesson 2: Mirrors

# Parabolic mirrors and real images

Parabolic Mirrors and Real Images. Created by Sal Khan.

## Want to join the conversation?

• How can you distinguish whether an image is real or virtual?
• Andrew's answer is great, but don't use it to start any fires :-)

I just want to add one thing to the talk about virtual images versus real images. The virtual image is construct of our brains. The virtual image is the way our brain interprets the light it is receiving (or better yet the signals from our optic nerves which receive the light). While this could be said about the real image as well, there actually is light at the spot of the real image. A virtual image is like a mirage; while a real image is...well...real.
• Why are the rays coming from the sun assumed to be parallel to each other?
• The sun and the earth are 150 mill. sq. km. away, this means that any rays which are not parallel to each other, be it converging or diverging ones, will just not reach earth but spread away, hence the rays coming from such a distance are taken to be parallel.
• is there ever a time when the glass that the mirror is made of causes total internal reflection and we see nothing???
• Since its a mirror, there wont be a case of total internal reflection. TIR arises only when light hits a transparent surface(while travelling from denser medium to the rarer medium) at an angle greater than its critical angle, where 100% of the light is reflected back into the same medium.
• At around -, Sal says that if a screen is put at the point where the two rays converge, an image will be projected on the screen. This is where I kind of get confused. So what he's saying is that if I stuck an every day object, say an eraser, in front of a parabolic, or concave, mirror, and piece of paper where the rays converge, then I should see an image of the eraser on this piece of paper? I am having a hard time grasping this concept.
• First you need to understand that concave and convex are SIMILAR to parabolic mirrors.
But other than that, you got the concept. The whole point is that light rays that hit the parabolic mirror (parallel to the principal axis), will reflect and go to the Focus. While light that passes through the Focus will reflect and travel parallel to the principal axis. ( vise versa)

Thats how the light rays converge and hence form a real image. :)
• what happens when the light shines 90 degrees straight backwards onto the parbolic mirror of the car? what happens to those light
• Like if the light would shine into the exact middle of the mirror? Like in the middle of the curve? Well then I believe it would act like a normal mirror because the normal and all.
• What is the between real and virtual images? I can't seem to tell if there is a difference or not.
• REAL-It can be formed on the screen light rays actually meet after refraction
VIRTUAL-It cannot be formed on screen. Light rays do not meet after refraction
• Is this how solar ovens work?
• yes the solar cooker uses this property.
it has a converging lens which converges all the sun rays to a particular point where the food which has to be heated is kept or we can say that food is placed in the focal point of the mirror.
• So a parabolic mirror, if you zoom in really really really far, will just look like geometric sides (not round) -which is why light reflects at different angles?
In other words, light is reflecting off of a bunch of flat but small surfaces?