Convex and Concave Lenses

hi friends if you place an object like this pencil in front of a convex lens do you know where the image will be formed what will be the properties of the image or if you replace it with a concave lens now what is the image gonna look like actually I don't need this pencil because in this video I am going to be the object and I'm going to go and stand in front of the convex and concave lens and let's see where my image is formed I'm going to make the topic of image formation for

spherical lenses really easy for you and as usual we'll finish off this video with our top three questions on this topic remember we have light ray rules for spherical mirrors and we use these rules to draw a ray diagram and obtain the image if you haven't watched my video on spherical mirrors do check it out similarly we have light ray rules for spherical lenses in this video we'll be looking at these rules and how to apply them for our ray diagrams let's start with the convex lens the convex lens is a converging lens for image

formation we need to draw the lens accurately let me show you a simple trick for that draw a line representing the principal axis mark the optical center oh roughly in the center of the line using a ruler mark two points to the right of o f2 is let's say at 2 centimeters from o and 2 f2 is double the distance it's at 4 centimeters from o similarly on the left side mark F 1 & 2 have fun using the same distance o is the optical center of the lens and f1 and f2 are the two

principal foci of the lens now draw the convex lens you can roughly sketch the lens or use a compass to draw the two arcs of the convex lens make sure that the lens is thin don't draw thick lens now let's take a look at the rules for image formation of a convex lens light is a very visual topic so I would suggest you to keep pausing the video and draw the ray diagrams in your notebook then you'll get a much better feel of what is going on there are three rules for the convex lens rule

one a ray of light which is parallel to the principal axis after refraction passes through the focus on the other side of the lens rule two a ray of light passing to the optical center of the lens goes straight it emerges without any deviation rule three a ray of light passing through the focus after refraction becomes parallel to the principal axis let's place the three rules that we have learnt for convex lens on our concept board can you see that rule three is exactly the opposite of rule one it's due to reversibility of light remember

when drawing the ray diagram you need to draw two rays from a point on the object so you need to choose two out of the three rules you can choose any two rules that are convenient for your diagram and as I promised I am going to be the object and I'll go and stand in front of the convex lens and you need to apply these rules and find my image so let's go ahead and try it out Here I am on the principal axis I am far away from the convex lens I'm going to walk

and stand on different places on the principal axis and as you'll see my image will change based on my position so let's start with case one I am standing beyond two one you need to consider the raise from the top of the object so from my head remember we need to select two out of the three rules let's use rule 1 the Ray which is parallel to the principal axis after refraction by the lens passes through the focus so this is our first ray here for the second rate let's use rule 2 the Ray which

passes through the optical center goes straight through undeviated so what does the intersection of these two light rays represent that's right the image it's the image of my head the top of the object because we consider the light rays from the top of the object so where is the feet of my image it's on the principal axis so you don't need to draw the light rays for the bottom of the image simply extend the top of the image to the principal axis and we get the bottom of the image so as you can see we

found the position of my image here what are the properties of an image there are three points to consider here first is the image real and inverted or is it virtual and upright second is the image magnified diminished or is it the same size as the object and third what is the position of the image so what are the properties of the image here for case one that's right the images real and inverted its diminished and as you can see the image is formed between f 2 and 2 f 2 you'll find similar diagrams in

your textbook of course I won't be the object there the object is usually shown with an up arrow since the object is upright and marked as a be the image is also shown with an arrow and it's represented as a - B - as you can see the arrows inverted here because the image is an inverted image now let me walk closer to the lens I'm going to stand at 2 F 1 for our case 2 once again we need to use 2 out of the 3 rules for the first tree let's use rule 1

the ray of light parallel to the principal axis after refraction passes through the focus of the lens for the secondary we can use rule to the Ray passing to the optical center of the lens goes straight through undeviated what are the properties of the image here can you see the symmetric nature of the diagram I'm standing at 2 F 1 and my image is formed at 2 F 2 the image height is the same as the object height that's my height and can you see that the images real and inverted it's a real image because

it's formed by the intersection of the Rays if you try drawing the diagram for this case you might find that the image is not exactly at 2 F 2 now this can happen if your foci F 1 and F 2 and the points 2 F 1 and 2 F 2 are not exactly equal distant from each other or the Rays are not accurately drawn so to avoid such errors an important exam tip for this case - is to use the cheating trick which is first draw the image and then you draw the Rays since we

know all the properties of the image we can easily draw it here it's a real and verted image and this image is going to be exactly at 2 F 2 and the image height is exactly equal to the object height so once you've drawn the image you can go ahead and draw the Rays now I'm going to move closer to the lens so we have our case 3 here where the object is between f1 and 2 f1 I would suggest you to pause the video here draw the ray diagram and try to find the image

yourself also write down the properties of the image formed so what's the nature of the image you got that's right the correct answer is it's a real and inverted image that's magnified here's the ray diagram and as you can see the position of the images beyond 2 f2 next I'll move ahead and stand at the focus of the convex lens again we need to apply 2 out of the 3 rules this is an interesting case because as you can see the refracted rays are parallel the refracted rays never meet so where is our image formed

the images faraway formed at infinity the image is real and inverted and it's highly magnified now let's look at the final case where I move really close to the lens the object that is me is between the optical centre and the focus of the lens let's use rule 1 and rule 2 to draw the light rays from the top of the object as you can see the refracted rays here are divergent so we need to them backwards to obtain our image Wow can you see the images magnified here so what are the properties of the

image the image is virtual and erect it's magnified and it's formed behind the object do you know a use of convex lens for this case that's right it's used as a magnifying glass like the one used by Detective Sherlock Holmes now that we are done with the convex lens let's go ahead and take a look at the other type of spherical lens concave lens the concave lens is a diverging lens for image formation we need to draw the concave lens accurately similar to the convex lens draw a line representing the principal axis again we need

to mark the points optical centre O and F 1 to F 1 and F 2 and to F 2 all these points are at the same distance from each other in this diagram the distance is 2 centimeters note that in a concave lens usually the F 1 and F 2 are on the opposite side compared to a convex lens so F 1 and 2 F 1 are on the right side and F 2 and 2 F 2 are on the left side you might find different conventions in different textbooks now let's draw the concave lens

you can roughly sketch the lens or use a compass to draw the two arcs of the concave lens make sure the lens is thin don't draw thick lens now let's take a look at the rules for image formation of a concave lens similar to the convex lens there are three rules for a concave lens rule 1 a ray of light which is parallel to the principal axis after refraction appears to be coming from the focus Rule two a ray of light going through the optical center of a concave lens go straight through without being deviated

rule three a ray of light going towards the focus after refraction becomes parallel to the principal axis let's place the three rules that we have learnt for concave lens on our concept board similar to the convex lens can you see that rule three is exactly the opposite of rule one it's due to reversibility of light to draw the ray diagram you need to select two out of the three rules once again I'm going to be the object and this time I'll stand in front of the concave lens and you need to apply these rules and

find my image so are you ready let's go ahead and try it out Here I am on the principal axis at a certain distance from the concave lens again we need to take the light rays from the top of the object so from my head remember we need to use two out of the three rules for the first ray let's use rule 1 the ray that is parallel to the principal axis after refraction if we extend the refracted ray backwards it appears to come from the focus for the second ray let's use rule 2 the

rate going towards the optical center goes straight through without being deviated now where do these two refracted rays meet since there are divergent rays we need to produce them backwards and the point where they meet is the image of my head because we took the Rays from the top of the object so where are the feet of my image we don't need to draw Ray's for that you simply extend the image that is the top of the image to the principal axis and you get the bottom of the image so as you can see the

image is formed between me the object and the concave lens what are the properties of the image here that's right the image is virtual and it's upright the image is diminished and it's formed between the object and the lens now let me walk closer to the lens if you draw the ray diagram can you see that we get the image having the similar properties the image is virtual and upright the image is diminished and it's formed between the object and the lens in fact the concave lens is the easy one in the syllabus because no

matter where the object is placed the image properties will always be the same but remember the convex lens had many different cases based on the location of the object now that we are done with the topic of image formation for spherical lenses are you ready for the top three questions on this topic coming up for you right now friends practice makes you perfect so try solving these questions and do let me know your answers in doubt by putting it in the comments below I promise to reply to all your comments as soon as possible so

I'm going to disappear and you pause the video here and give these questions a shot and to watch more science and maths videos like these do check out my website monitor academy.com and if you haven't subscribed to my youtube channel already go hit the subscribe button right now and remember to like and follow my facebook page thanks for watching you