Answer
Hint: Use the formula for the image of the lens i.e. $\dfrac{1}{v}-\dfrac{1}{u}=\dfrac{1}{f}$, where u is the position of the object from the lens and v is the position of the image formed from the lens (according to sign convection). Then check what happens when the object is placed at the focus of a concave lens and at the focus of a convex lens.Complete step by step answer:Lens is a transparent medium bounded by two refracting surfaces, such that at least one surface is spherical. There are mainly two types of lenses- convex lens and concave lens. Convex lens is also called converging lens and concave is also called diverging lens. Convex lens is thick at the middle and the thickness decreases until the end. Concave lens is thinnest at the middle and the thickness increases until the end. When light from an object passes through these lenses they refract the light and form an image of that object. This is given by the formula $\dfrac{1}{v}-\dfrac{1}{u}=\dfrac{1}{f}$ ……(1) , where u is the position of the object from the lens and v is the position of the image formed from the lens (according to sign convection). F is the focus of the lens and f is the distance of the focus from the lens. For a convex lens, the value of f is taken positive and for concave lens, the value f is taken negative. Now we will see what this focus of the lens is. Suppose an object is at the focus of a concave lens, then u = -f. Substitute the value of u in equation (1). $\dfrac{1}{v}-\dfrac{1}{(-f)}=\dfrac{1}{(-f)}\Rightarrow \dfrac{1}{v}=-\dfrac{2}{f}$ $\Rightarrow v=-\dfrac{f}{2}$ This means that the image of the object will be formed at a distance $\dfrac{f}{2}$ of behind the lens.
When viewing an object through a convex lens, the observer will always see an upright, smaller version, for all positions of the object.
This can be demonstrated as follows.
Object at more than `2F`
This means that the object is further away than two principal focal lengths from the concave lens.
The image is smaller, upright, virtual and between the principal focal point on the object side and the concave lens.
Object at `2F`
When the object is at two principal focal points from the concave lens, you get the following ray diagram:
The image is smaller, upright, virtual and between the principal focal point on the object side and concave lens.
Object between `2F` and `F`
Placing the object between two focal points and one focal point produces the following ray diagram:
The image is smaller, upright, virtual and between the principal focal point on the object side and the concave lens.
Object at `F`
When the object is placed at the focal point of the concave lens, you get the following ray diagram:
The image is smaller, upright, virtual and between the principal focal point on the object side and the concave lens.
Object between `F` and concave lens
When the object is placed between the principal focal point and the lens, the following occurs:
The image is smaller, upright, virtual and between the principal focal point on the object side and the concave lens.
Conclusion
We can see from the above that for every position of the object the image created is always.
SMALLER
UPRIGHT
VIRTUAL (can not be projected on a screen)
Between principal focal point and the concave lens
All images through a concave lens will look smaller and closer.
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