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Lesson 3: Lenses

# Concave lenses

Concave lenses, with their inward curve, create divergent light rays. When a ray parallel to the principal axis hits the lens, it refracts outward as if coming from the focal point. Another ray, passing through the lens center, doesn't refract. These rays form a virtual, upright, and smaller image, appearing closer to the lens than the actual object. Created by Sal Khan.

## Want to join the conversation?

• what is the difference between reflecton and refraction
• Reflection is when a ray of light bounces off an object (think mirrors). Refraction is when a light ray transmits through an object but the ray's path is bent, exiting the object at a different angle from what it entered the object at.
• what if the lens is not symmetrical? is it even possible to have a non symmetrical lens? if it was possible, what would happen?
• yes, this type of lens is possible. But every lens follows same laws of refraction of light .
There are many possible lenses of this type like, cyclindrical lens, plano-concave, plano-convex, concavo-convex, convexo-concave, etc.
• What is the difference between refraction and diffraction? is there is a difference in angle of light change or velocity? Thanks.
• Refraction is the change in direction of waves which occurs when waves travel from one medium to another. Refraction is always accompanied by a wavelength and speed change. Diffraction is the bending of waves around obstacles and openings. The amount of diffraction increases with increasing wavelength.
• At , why does sal use a ray going through the middle that doesn't get refracted instead of one going through the focal point
• He could have drawn a line toward the right focus. This would have exited the lens parallel to the principal axis. If you follow that path back through the lens like he did with the one that shoots up to the right, you would find it intersects the same point. My teacher pointed out that drawing a third ray is a good way to be sure you haven't messed up somewhere, and through the center is pretty hard to mess up.
The thing to remember here is that the foci of a concave lens work opposite of a convex lens. Parallel rays entering the lens exit directly away from the near focus, and rays that move toward the far focus exit parallel.
• as the refractive index of concave mirror is higher than that of air, why do the rays not converge after being refracted? why r the diverged although the lens is a denser medium?
• Very very good question dude.
When rays of light enter the lens , due to change in absolute refractive index , they get converged. But as they exit the lens , once again they enter a rarer medium and thus diverge . Now the question arises that why don't the refracting forces equalize each other? This is because in a plano-concave lens , the angle between the refracting surfaces is maximum (at the edges) and hence the diverging power is maximized .
• how do we know if an image is virtual or not?
• Virtual images do not have any real place in the physical world where the light rays converge/meet. So when you analyse a situation (say by a ray diagram), if you find that the rays meet at a point, that would be a real image. If they do not actually meet, but only seem to diverge from a point, it is a virtual image.
• What type of lens is used in contact lenses (e.g. hard contacts, soft contacts, etc.)?
• If you have myopia or nearsightedness, you would use diverging lenses(concave) to shift the focus of your eye lens backwards so that it can focus on the retina. Just the opposite goes for hypermetropia or farsightedness, in which you would use converging(convex) lens to bring the focus closer. In astigmatism, you would use cylindrical lenses. There are various other types of eye problems that require other shapes of lenses.
• So, why would a ray going through the left focal point not go through the concave lens parallel as it did in convex lenses? I drew it and it messed up the whole drawing.
• So basically, the properties of the image formed using a concave mirror is identical to the properties of an image formed by a convex lens, and vice-versa, right?
• Yes my friend, in practical use you are definitely correct. And always remember difference between mirror and lens.
(1 vote)
• Why doesn't the beam going through the center get refracted as it leaves the glass?
(1 vote)
• Cool question.
If you really zoom in, you can find that at the center of the lens, the two edges are parallel and that kind of acts like a parallel-sided glass slab. Well, you know, when light emerges out of a glass slab after refraction, the emergent ray is parallel to the incident ray, but the ray would be displaced laterally.

Since, we're assuming that the lens is very thin, this lateral displacement is very, very, very small and hence, we feel like it doesn't get refracted. (Actually, it does.)

But for simplicity's (and our) sake, the refraction is not shown. Or else it would've been way, way more complicated.

Hope this helps. :)