Light rays enter the eye through the cornea, a transparent dome-shaped tissue that is the eye's outermost layer. The cornea bends or refracts, light rays traveling to the pupil. The shape of the cornea determines how much of the light is bent and whether the image will be focused correctly on the retina at the back of the eye. Once the light has passed through both refraction layers, it converges into a single focal point onto a small area. This is where photoreceptors start transforming...

Voltage and current measurements using a standard voltmeter and ammeter alter the circuit being measured either by drawing or resisting the current flow, which introduces uncertainties in the measurements. Null measurements balance the voltages so that no current flows through the measuring device and, therefore, no alterations occur in the measured circuit.
Suppose the emf of a battery needs to be measured. If the battery is directly connected to a standard voltmeter, the measured quantity is...