Final answer:
The light will touch the bottom of the lake directly under the point of impact if it enters the water vertically. Therefore, the distance from the point of impact to where the light touches the bottom is equal to the depth of the lake, which is 10m.
Step-by-step explanation:
The distance that light travels underwater depends on the angle at which it enters and the refraction of light. Since light tends to bend when it enters a medium with a different speed like water, the actual path it takes is not linear. However, if we assume that the light enters the water vertically (at 90 degrees), the distance from the initial striking point to where the light touches the bottom of the lake would be equal to the depth of the lake, which is 10m. Thus the light would touch the lake bottom directly under the point of impact.
Learn more about Refraction of Light