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# Convex lens examples

Convex Lens Examples. Created by Sal Khan.

## Want to join the conversation?

• I thought Convex lenses always displayed Virtual images.
• • Convex mirrors always display virtual
• Concave lenses always display virtual.
• Concave mirrors can display real or virtual.
• Convex lenses can display real or virtual.
• Why is it that the ray that goes through the center of the lens is not refracted (~)?
• because it doesn't change direction.... it will however slow down when it enters the medium... and refraction is the change in direction caused by this slowing down, but as the wave is normal to the surface it slows down at the same instant across it's "front" and as such doesn't change direction.
• At , if a ray passes right along the principal axis and through the center of the lens, wouldn't it intersect with the ray that passes through the focus, since the ray that passes through the center of the lens doesn't get refracted?
• I think light along principal axis from the bottom of object is not possible because the bottom is not visible ... so light must be taken from above the principal axis... (according to me) :D

hope this helps
• Do electrons absorb light ? Do they cause refraction?
• Electrons can absorb photons of light but this is not the reason for refraction of light.

Light is an oscillation of electromagnetic field, this interacts with the electric charges of the particles in the material causing an oscillation in the materials electromagnetic field. The two oscillations interfere with each other causing the the combined field oscillation to propagate slower than the speed of light in a vacuum and change direction.

When light exits the refractive material the interference no longer occurs so the light resumes its normal apparent velocity and direction.
• At , How does Sal tell that the image formed is inverted or erect?
• he showed 2 rays coming from the top point of the arrow, when they converge below the principle axis then it is an inverted image and when it it converges above the principle axis(usually in cases of virtual images) then it is an erect image.
• I understand how to draw the ray diagrams and how to solve problems asking whether the image is real or inverted, bigger or smaller, etc. However, my professor likes to ask questions that are very in depth and realistic. (Also, I am genuinely curious). I do not understand what you mean when you say "we will not see any image" or "we will see an inverted/real image, that is bigger" versus "we will see a virtual/upright image that is smaller", etc, in more realistic terms. In real life, like let us say that I am far sighted and I need glasses that allow me to, let's say, read a book. I would need such a convex lens, right. In this scenario, can you explain how/what it means for the image to be at the focal point, or between my center of curvature and focal point? How can I see the image inverted if I am reading a book? That has never happened to me… Nor have I seen things smaller. I am not sure if the question makes sense, but essentially I am asking if you could give a real life scenario of something that is AT the focal point or IN THE MIDDLE of the center of curvature and focal point, etc. Thank you!
• Well, when you are prescribed lenses for glasses, it is such that the rays are slightly bent towards or away from each other, but not that they form a real image or something on your eye.So, when the rays come, since they are not parallel, they do not converge at the focus of your eye's lens.
If you are farsighted, your eye is smaller.Farther objects form images on your retina, but near objects do not as their images form at a point behind your eye.Since you are prescribed convex lens, they converge some of the light from nearby objects(which is why you always wear them while reading,etc.) and this is again converged, but at a closer on another point in your eyes.This point is your retina.So, you can see clearly.
If you are nearsighted, your eye is larger.Closer objects form images on your retina, but farther objects do not as their images form at a point before your retina.Since you are prescribed concave lens, they diverge some of the light from farther objects(which is why you always wear them) and this is again converged, but farther on another point in your eyes.This point is your retina.So, you can see clearly.
Hope you understand and hope this helps!
• how can a lens give a vitual image?.....cannot we put a screen in where the magnified image is formed?...in the last example?
• No. Try it with a magnifying glass. Instead of looking into it with your eye to see the object you are trying to enlarge, put a screen where your eye would be, and look at the screen. Do you see an image?
• In the fourth case why didnt the second light ray get refracted..? at
• The light ray doesn't get refracted because it passes through the optical centre (o)
(1 vote)
• if the medium on either side of the lens is changed then what effect would it have on the focal length on the lens ???
• Lens change its focal lenght in the following ways--->
1.If the medium is dense then the focal lenght increases, due to the change in refractive index being lowered
2.If the medium is rare then the lens focal lenght decreases due to the change in refractive index being increased
it will also show its effect on the power of the lens.
power of the lens is given by the formula u/f.
in this formula u is the refractive index of the surrounding medium and f is the focal length of the lens in the surrounding medium.
note that a medium can have a different refractive index for different color of light rays.
hope that this would help.
(1 vote)
• Hyperopia, also known as farsightedness, is a defect of vision caused by an imperfection in the eye, (often when the eyeball is too short or the lens cannot become round enough), causing difficulty focusing on near objects, and in extreme cases causing a sufferer to be unable to focus on objects at any distance.This is my question,Is the "convex lens", can be a treatment in this eye defect called"hyperopia"?