Ch\u01b0\u01a1ng 7 – B\u00e0i 44: Thuy\u1ebft l\u01b0\u1ee3ng t\u1eed \u00e1nh s\u00e1ng. L\u01b0\u1ee1ng t\u00ednh s\u00f3ng – h\u1ea1t c\u1ee7a \u00e1nh s\u00e1ng<\/a><\/blockquote><\/iframe>