Question

a convex lens is made by a medium of u1 and u2 is the refractive index of surrounding medium. then what is the condition to get focal length negative..?​

Answer 1

The condition to get focal length negative is when object is behind the mirror.

Explanation:

• F is referred to as the focal length.  
• The positive focal length is for the concave lens.
• The negative is for the convex lens.
• The condition in which the image is behind the mirror, it causes the negative focal length of the object.

• The negative or positive focal length is also used to find the distance of an object.

Can focal length of convex lens be negative

https://brainly.in/question/7494408

Answer 2

For negative focal length, $n_l<n_m$.

Explanation:

Lens maker`s formula,

$\frac{1}{f} =\frac{n_l-n_m}{n_m} (\frac{1}{R_1} -\frac{1}{R_2} )$

Here n_l is the refractive index of the lens and n_m is the refractive index of the medium and convex lens the radius R1 is positive since it is measured from the front surface and extends right to the center of curvature. The radius R2 is negative since it extends left from the second surface.

Now if refractive index of the medium is greater than refractive index of the lens then the focal length will be negative.  

Thus, to get the negative focal length $n_l<n_m$.

#Learn More: Lens maker`s formula.

https://brainly.in/question/13182605