Final answer:
Interstellar dust clouds are visible because they reflect starlight and can absorb and re-emit light, particularly in the infrared spectrum. They also block light from stars, which contributes to their visibility. To study these dust clouds, especially dense ones like molecular clouds, infrared or radio telescopes are utilized as they can see through the cloud's opacity.
Step-by-step explanation:
Clouds in space, such as interstellar dust clouds, do not emit their own light, but they can still be seen through various phenomena. The correct answer to the question 'How can these clouds' material be seen if they don't emit their own light?' is that they reflect starlight and can absorb and re-emit light in different parts of the electromagnetic spectrum, such as the infrared. On some occasions, these clouds block the light from distant stars, making them appear dimmer. Such clouds can also cause a reddening effect, making stars seem redder due to the scattering of light by particles within the cloud. Moreover, interstellar dust and gas are responsible for the absorption of light, and astronomers have concluded that widely scattered solid particles must cause the dimming of starlight. Although most interstellar clouds are opaque to visible light, they can be studied using infrared or radio telescopes which are able to penetrate these dense clouds, providing insight into regions where stars and planets are forming.