Doesn't the refractive index affect the convex and concave lens? Example, what if you use a diamond convex lens and a glass convex lens with the same focal length. Don't they have a difference while refracting?
Answers
Answer:
Explanation:
Lenses bend light that passes through them. The direction and amount that the light bends depends on the curvature of the lens, the material the lens is made of, and the material in which the lens is immersed (for now, we’ll assume this is just air). If both sides of the lens curve outward, it is called a converging lens, and it will bend light from distant objects inwards toward a single point, called the focal point.
Figure of converging lens with light rays focussed on the opposite side
Figure of converging lens with light rays focussed on the opposite side
If both sides of the lens curve inward, it is called a diverging lens, and light from distant objects will bend outwards. Because the light is not being bent toward a single point, the focal point is not as obvious as it was in the case of the converging lens. We have to take the bent rays, and follow them back to the side of the lens that the light came from to make them come together and find the focal point. That means that the focal point is on the same side of the lens as the light rays were coming from.
Figure diverging lens with light rays diverging on the opposite side
Figure diverging lens with light rays diverging on the opposite side
In actuality, there are two focal points for every lens, the same distance from the lens, on opposite sides. The distance from the lens to the focal point is called the focal length. For converging lenses, the focal length is always positive, while diverging lenses always have negative focal lengths. However, these conventions are arbitrary, and physicists could just as easily have made the signs opposite.
Figure of converging lens with labeled negative and positive focal lengths
Figure of converging lens with labeled negative and positive focal lengths
Thin lens rules and sign conventions
Now that we know how to find the focal point of a lens using a distant object, we can see what happens to light rays from objects that are closer to the lens. Let’s say we have a cat standing on one side of a converging lens. We know that there are two focal points, one on each side of the lens, and that if we take the focal length of the point across the lens from the cat as positive, then the one on the same side as the cat is negative.
In this case, the image of the cat will be across the lens from the actual cat, and it will be upside-down. Just as with the focal point, if the image is on the opposite side of the lens from the cat, the distance from the lens to the image will be positive.
Figure of cat with positive object distance, negative focal point, lens, positive image distance to inverted image of cat
Figure of cat with positive object distance, negative focal point, lens, positive image distance to inverted image of cat
What if the cat is closer to the lens than the focal point is? That is, what if the object distance is smaller than the focal length? The image will be on the same side of the lens from the object, and will be upright. The image will also be larger than the object. That means that the image distance will be negative.
Figure of large upright image of cat, negative focal point, smaller cat, lens, positive focal point
Figure of large upright image of cat, negative focal point, smaller cat, lens, positive focal point
How about a diverging lens? This time, the image is not affected by whether the object is inside or outside the focal point. The image will always be on the same side of the lens as the object, upright, and smaller than the object. In this case the image distance for the diverging lens is negative.
Figure of focal point, cat, small image of cat, diverging lens, focal point
Figure of focal point, cat, small image of cat, diverging lens, focal point
Magnification refers to a change in size of the object. If the magnification is greater than one, the image is larger than the object, but if the magnification is smaller than one the image is smaller than the object. For example, if the magnification is one half, then the image appears to be half the size of the object. The sign of the magnification tells us the orientation of the image. If the sign is positive, then the image is upright. If the sign is negative, then the image is upside-down. In the examples above, we can see that amount by which an object will be magnified changes depending on its distance from the focal point.