$A$ satisfactory photographic print is obtained when the exposure time is $10 \, s$ at a distance of $2 \, m$ from a $60 \, cd$ lamp. The time of exposure required for the same quality print at a distance of $4 \, m$ from a $120 \, cd$ lamp is $... \, s$.

$A$ ray of light falls on the surface of a spherical glass paperweight,making an angle $\alpha$ with the normal,and is refracted in the medium at an angle $\beta$. What is the angle of deviation of the emergent ray from the direction of the incident ray?

$A$ camera lens is set at $f/2$ with an exposure time of $1/100 \ s$. If the lens is set at $f/4$,what will be the correct exposure time?

In a laboratory,a material is created with a negative refractive index. $A$ light ray enters from medium $1$ into this material (medium $2$). Which of the following diagrams correctly represents the path of the ray?

$A$ $2\,m$ long scale with a least count of $0.2\,cm$ is used to measure the locations of objects on an optical bench. While measuring the focal length of a convex lens, the object pin and the convex lens are placed at $80\,cm$ mark and $1\,m$ mark, respectively. The image of the object pin on the other side of the lens coincides with the image pin that is kept at $180\,cm$ mark. The $\%$ error in the estimation of focal length is

$A$ small piece of wire bent into an $L$ shape with upright and horizontal portions of equal lengths,is placed with the horizontal portion along the axis of a concave mirror whose radius of curvature is $10 \, cm$. If the bend is $20 \, cm$ from the pole of the mirror,then the ratio of the lengths of the images of the upright and horizontal portions of the wire is: