$A$ man wishing to get a picture of a Zebra photographed a white donkey after fitting a glass with black streaks onto the objective of his camera.

Physical processes are sometimes described visually by lines. Only the following can cross.

If the luminous intensity of a $100 \, W$ unidirectional bulb is $100 \, \text{candela}$, then the total luminous flux emitted from the bulb is ....... $lumen$.

An optical bench has a $1.5 m$ long scale having four equal divisions in each $cm$. While measuring the focal length of a convex lens,the lens is kept at the $75 cm$ mark of the scale and the object pin is kept at the $45 cm$ mark. The image of the object pin on the other side of the lens overlaps with the image pin that is kept at the $135 cm$ mark. In this experiment,the percentage error in the measurement of the focal length of the lens is. . . . .

As shown in the figure, a parallel beam of light is incident on the upper part of a prism of angle $1.8^{\circ}$ and material of refractive index $1.5$. The light emerging from the prism falls on a concave mirror of radius of curvature $40 \,cm$. The distance of the point from the principal axis of the mirror where the light rays are focused after reflection from the mirror is:

Five lumen/watt is the luminous efficiency of a lamp and its luminous intensity is $35$ candela. The power of the lamp is.......$W$